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April 25
Bacterial Canker of Tomato

Recently, this disease was observed in a greenhouse in Indiana.  This article will serve as a review of this important disease.

The symptoms of bacterial canker vary considerably.  In most cases, the edges of the leaves may turn yellow and/or brown.  That is, the margins of the leaves may become chlorotic and/or necrotic.  This symptom, which is sometimes known as ‘firing’, may be more common in a field situation than in a greenhouse.  Tomato plants may wilt as a result of bacterial canker.  The inside of the stem of affected plants may be discolored brown.  The fruit may have bird’s-eye spots-this symptoms is more common in field outbreaks.  In the greenhouse where this disease was recently observed, adventitious root development was observed on the stems of affected plants.  That is, the stems may develop a ‘bumpy’ appearance where extra roots are starting to develop. However, this symptom may also develop from stresses other than bacterial canker.

The bacterium which causes bacterial canker of tomato may survive in seed, crop debris, volunteer tomatoes and equipment such as wooden stakes.  The pathogen may spread from plant to plant by splashing.  This is most likely during transplant production in the greenhouse.  Once infected, tomato plants may continue to develop symptoms, which may give the appearance of spread in the field. 

The most important factor in managing bacterial canker of tomato is to avoid seed contaminated with the pathogen or transplants that have symptoms.  Heat treatment of seed to reduce contamination is possible; see the Midwest Vegetable Production Guide for Commercial Growers 2016.   Use only new or sterilized planting stakes, transplant trays and other planting equipment.  The use of copper and mancozeb products for management of bacterial canker of tomato is more effective in greenhouse transplant production than in the field. 

Bacterial canker of tomato can become a very serious disease.  If you believe that your tomatoes may suffer from bacterial canker, be sure to get an official diagnosis. 

 ​fig 1xx.JPG
Figure 1:  Necrotic and chlorotic margins of tomato leaves caused by bacterial canker of tomato.

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Figure 2:  Bacterial canker may cause a discoloration of the interior of the stem.

fig 3xx.jpg

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Figure 3:  Occasionally, bacterial canker may cause a adventitious roots that appear as a ‘bumpiness’ on the stem of the tomato plant. ​ 

March 15
Lettuce Drop

Cool season crops such as lettuce are becoming a more popular crop among Indiana greenhouse/high tunnel growers. One of the most important diseases of lettuce is known as lettuce drop.  

The symptoms of lettuce drop are often noticed after the thinning stage, early in the crop development. The early symptoms may include browning of leaves. Later on in the crop development, the outer most leaves of the lettuce plant may wilt. As the disease become more severe, inner leaves may become infected. Eventually, the entire plant may collapse. The plant often has white mold on the leaves and dark irregular fruiting bodies may be observed (Figure 1). The dark fruiting bodies are known as sclerotia.

Two different organisms may be responsible for lettuce drop. Sclerotina sclerotiorum and S. minor

Lettuce drop caused by S. sclerotiorum requires a chilling period (52 to 59 F) for the sclerotia to turn into mushrooms smaller than a dime. These mushrooms then produce spores that may cause disease of lettuce if there is adequate leaf wetness. S. minor, in contrast, is seldom caused by spores from a mushroom. Instead, lettuce drop caused by S. minor is often caused by direct germination of the sclerotia in the soil.

In summary, S. sclerotiorum usually causes disease through mushrooms that sprout after a sufficient chilling period. Thus, mushrooms some distance away from the lettuce could cause S. sclerotiorum infection. S. minor mostly causes disease through direct germination of sclerotia in the soil. S. minor infections, therefore, are most likely the result of close contact with soil infested with sclerotia.

It is my opinion that lettuce drop in Indiana greenhouses is more likely caused by S. sclerotiorum. Part of the reason for this belief is that S. sclerotiorum is the same organism responsible for white mold of tomato. Tomato is a common crop in Indiana greenhouses and is commonly rotated with lettuce. Also, sclerotia produced by S. sclerotiorum tend to be larger and more irregularly shaped than those produced by S. minor. The sclerotia in figure 1 are from Indiana and are most likely S. sclerotiorum.

Fungicides may help to reduce the incidence of lettuce drop. Products labeled and either approved or silent on greenhouse use include: Botran 75W®, Cannonball®, Fontelis®, Rovral® and Switch®. The product Contans®, which is organically approved in many systems, is a hyperparasite of both S. sclerotiorum and S. minor. Contans must be worked into the soil. 

Other cultural methods of disease reduction include: Deep plow soil to get sclerotia well under the soil surface; Long crop rotations with a non-host may help to reduce disease; Anything which can be done to increase ventilation/decrease leaf wetness will help to reduce disease severity of lettuce drop caused by S. sclerotiorum.

Please see the Midwest Vegetable Production Guide for Commercial Growers 2016 (ID-56) for more information <mwveguide.org>. 

 ​LettuceDropPhotoResized.jpg
Lettuce drop symptoms include browning of tissue, white mold growing on plant and the irregular, dark fungal structures known as sclerotia.  (Photo:  Wenjing Guan).  

March 02
Updates to the MW Vegetable Production Guide for Commercial Growers 2016

The table below gives the changes that have been made to the on-line version of the ID-56 as of this date.  If you have purchased, or will purchase, a hard copy of this guide, please make these changes.  If you use the  ID-56, these updates will already have been incorporated.  

Note, many of the changes have to do with the newly registered Orondis products from Syngenta.  Please contact me with any questions.

 

Page

Comment

42

Move Oberon from the label prohibits greenhouse use to the column label silent on greenhouse use.

43

Move Quintec from the column label silent on greenhouse use to label prohibits greenhouse use. 

74

Add row…

Orondis Opti/oxathiapiproplin (U15); chlorothalonil (M)/medium-high/U15, M

74

Add row….

Orondis Ridomil Gold SL/oxathiapipropilin (U15); mefenoxam (4); /medium-high/U15, 4

74

Modify row…

Orondis Ultra/oxathiapipropilin (U15); mandipropamid (40)/U15, 40

74

oxathiapipropilin under common name, is one word.  Same row as Orondis Ultra.

108

Under downy mildew…add

Orondis Opti     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

108

Under downy mildew….change Orondis Ultra entry to….

Orondis Ultra     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

109

Under Phytophthora…add

Orondis Opti     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

109

Under Phytophthora…add

Orondis Ridomil Gold SL   Follow rates given on each container of multi-pack.  5-Day PHI.

Apply as a mix of both products in multi-pack to soil only. 

109

Under Phytophthora….change Orondis Ultra entry to….

Orondis Ultra     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

 

 

109

Add as recommended product under powdery mildew, “Vivando at 15.4 fl. oz. per acre.  0-Day PHI Comment:  Must be in possession of supplemental label.”

111

Add-Vivando (12/0)/metrafenone (U8)/G under powdery mildew/Comment: must be in possession of supplemental label. 

111

Add-Forum 4.18 SC (12/0)/dimethomorph (40)/add G for downy mildew and G for Phytophthora/comment: Do not alternate with Revus.

111

Revus-add ) in second column

111

Change G to F under powdery mildew for Inspire Super, Luna Sensation, Monsoon

111

Change G to P under powdery mildew for Pristine. 

111

Add row…

Orondis Ridomil Gold SL (48/5)/oxathiapipropilin (U15), mefenoxam (4)/‘G’ under Phytophthora blight/Apply as mix of both products in multi-pack to soil.

111

Modify row…

Orondis Ultra (4/0)/oxathiapipropilin (U15), mandipropamid (40)/’G’ under downy mildew, ‘G’ under Phytophthora blight/ Apply as tank mix of both products in multi-pack.

124

Under Phytophthora…add

Orondis Opti     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

124

Under Phytophthora…add

Orondis Ridomil Gold SL   Follow rates given on each container of multi-pack.  7-Day PHI.

Apply as a mix of both products in multi-pack to soil only. 

124

Under Phytophthora….change Orondis Ultra entry to….

Orondis Ultra     Follow rates given on each container of multi-pack.  0-Day PHI.

Apply as tank mix of both products in multi-pack. 

129

Under Buckeye rot/Phytophthora…add

Orondis Opti     Follow rates given on each container of multi-pack.  0 Day PHI.

Apply as tank mix of both products in multi-pack. 

129

Under Buckeye rot/Phytophthora blight…add

Orondis Ridomil Gold SL   Follow rates given on each container of multi-pack.  7-Day PHI.

Apply as mix of both products in multi-pack to soil only. 

129

Under downy mildew….change Orondis Ultra entry to….

Orondis Ultra     Follow rates given on each container of multi-pack.  1-Day PHI.

Apply as tank mix of both products in multi-pack. 

130

Under late blight…add

Orondis Opti     Follow rates given on each container of multi-pack.  3-Day PHI.

Apply as tank mix of both products in multi-pack. 

130

Under late blight add….

Orondis Ultra     Follow rates given on each container of multi-pack.  1-Day PHI.

Apply as tank mix of both products in multi-pack.

132

Add row…

Orondis Opti (4/3)/oxathiapipropilin (U15), chlorothalonil (M)/’VG’ under late blight, ‘VG’ under Phytophthora blight/Apply as tank mix of both products in multi-pack. 

132

Add row…

Orondis Gold 200 (48/28)/oxathiapipropilin (U15), mefenoxam (4)/‘VG’ under Phytophthora blight/Apply as mix of both products in multi-pack to soil.

132

Modify row…

Orondis Ultra (4/1)/oxathiapipropilin (U15), mandipropamid (40)/’VG’ under late blight, ‘VG’ under Phytophthora blight/Apply as tank mix of both products in multi-pack.

132

Table footnotes:   add under fungicide rating….

‘VG=very good’

175

Add….under late blight….

Orondis Opti    Follow rates given on each container of multi-pack. 7-Day PHI.  Apply as tank mix of both products of multi-pack. 

175

Add….under late blight….

Orondis Ultra     Follow rates given on each container of multipack.  14-Day PHI. Apply as tank mix of both products of multi-pack. 

February 26
New Fungicide for Vegetables

I would like to announce the release of a new fungicide, Orondis from Syngenta.  It is a good product and should help commercial vegetable growers in combating downy mildew of cucurbits, Phytophthora blight of cucurbits, peppers and tomato, Buckeye rot of tomato and late blight of potato and tomato.  However, I also want to discuss Orondis because of the complicated way in which it is being released.  Be advised that the listing for Orondis in the MW Vegetable Production Guide for 2016 (ID-56) is incorrect.   Please see the on-line version of the ID-56 for the most current information.  

Orondis has a new active ingredient which does not appear in any other fungicide and a novel mode of action, FRAC code U15.   But you will not be able to purchase Orondis on its own.  It will be available as 3 different multi packs or co-packs. Each multi-pack will contain two jugs, each with a different active ingredient and mode of action.   The products in the multi-pack are intended to be used as a tank mix.  The correct use of the products will help to prevent the emergence of strains of the pathogens that are resistant to FRAC group U15.  See Table 1 below for details.

This blog discusses the use of Orondis products with cucurbits and solanaceous crops because I believe that Orondis should have the greatest impact on these crops.  However, many other crops are listed on the Orondis label.    For more details, check with Syngenta, your local chemical representative or me


 

 

Table 1:  Orondis products will be available in 2016 as multi-packs.  That is, growers will purchase a box with 2 different jugs.  Each jug contains a different fungicide.   Mix both products together as a tank mix in water for application.  Follow the rate information on each product.   Use the most restrictive REI and PHI* for each product in a multi-pack.  After using one of the Orondis products, alternate to a product with a different mode of action.  Do not apply a foliar application of an Orondis product after a soil application of an Orondis product.  That is, use soil or foliar applications of Orondis, but not both. 

Name of multi pack

Products in multi pack (REI)

Common name of a.i. (FRAC code)

Crops (PHI)

Rates

Orondis opti

Orondis Opti A (4)

Oxathiapipropilin (U15)

Cucurbits (0)

2-4.8 fl. oz/A

Pepper (0)

Tomato (0)

Potato (5)

1.6-4.8 fl. oz/A

Orondis Opti B (12)

Chlorothalonil (M)

Cucurbits (0)

2 pts/A cucurbits

Pepper (3)

1.5 pts/A

Tomato (0)

2 pts./A

Potato (7)

¾ pts/A

Orondis Ridomil Gold SL**

Orondis Gold 200 (4)***

Oxathiapipropilin (U15)

Cucurbits (0)

2.4-19.2 fl. oz.

 

Pepper (0)

Tomato (0)

Ridomil Gold SL (48)

Mefenoxam (4)

Cucurbits (5)

1-2 pts/A 

Pepper (7)

1 pt/A

Tomato (7)

1-2 pt/A

Potato (14)

3.2 fl oz./A

Orondis Ultra

 

Orondis Ultra A (4)

Oxathiapipropilin (U15)

Cucurbits (0)

2-4.8 fl. oz

Pepper (0)

Tomato (0)

Potato (5)

1.6-4.8 fl. oz/A

Orondis Ultra B (4)

Mandipropamid (40)

Cucurbits (0)

8 fl oz/A

Pepper (1)

Tomato (1)

Potato (14)

* PHI=Pre-harvest Interval in days; REI=Restricted Entry Interval in hours.

**Orondis Ridomil Gold SL may only be applied to soil.  

***Orondis Gold 200 is not labeled for potato.  

January 10
Organic Products for Management of Powdery Mildew of Pumpkins

In my last blog I discussed management of powdery mildew with conventional fungicides.  Here I would like to talk about powdery mildew management of cucurbits with organically approved products.  I will describe two studies, one with all organically approved products and a second with a combination of organic and conventional products.  All studies were conducted at the SW Purdue Ag Center in Vincennes, IN. 

The organic products discussed are defined as organic since they appear on the Organic Material Review Institute (OMRI).  There are other certifying agencies.  Be sure to check with your certifying agency before using any fungicide product.  As an example, the Champ DP product used in 2010 is listed by OMRI as approved.  However, Champ WP is not.  

In the 2010 study shown below, zucchini of the variety Raven F1 were planted in the certified organic plot managed at the SW Purdue Ag Center.  Organic products were applied using a CO2 backpack sprayer from 22 Jul to 31 Aug.   Each product was applied one time per week except for Oxidate which was applied twice weekly.  The reason Oxidate was applied twice a week is that the active ingredient, hydrogen dioxide, has little or no residue to remain on the plant surface after product has dried. 

Only the Champ and Milstop treatment had significantly less powdery mildew than the untreated check (Figure 1).   The Oxidate and Serenade Max treatments were not significantly different than the untreated control. 

By the criteria used in most agricultural trials, there was no significant differences in total yields.  However, for the yields of 1 Sep, there were differences at the 10% level (most agricultural studies require differences at the 5% level). On 1 Sep, the Champ treatment had 1,525 fruit per acre, significantly more than the Milstop, Oxidate or the untreated check.  The Champ and Serenade Max treatments were not significantly (data not shown). 

The copper product, Champ DP, outperformed all the other organically certified treatments in this trial.  It is important to note that the Oxidate treatment did not match the Champ treatment even when applied twice a week.  While Oxidate can disinfest the surface of plants of many plant pathogenic fungi, the absence of any residue makes it an inferior treatment in this situation. 

The second figure is from a cantaloupe study in 2012. The study was conducted in a conventional plot.  Fungicides were applied with hollow cone nozzles with 30 PSI using a 3-point hitch one row sprayer.  Saf-T-Side and Nordox are both organically certified.  Nordox is a cuprous oxide (copper) product.  Saf-T-Side contains petroleum oil.  The trial is published here.

 All of the treatments had significantly less powdery mildew than the untreated check.   The disease levels of the Saf-T-Side and Nordox treatments were not significantly from each other; the Nordox disease level was not different from the Rally or Pristine/Rally treatments.  The remaining treatments:  Bravo/Torino/Rally, Procure/Torino/Rally, Quinec/Torino/Rally and two rates of Merivon all had significantly less powdery mildew than the organic treatments or the untreated check.  

The untreated check was not significantly different in yield in lb/A than any other treatment except the Merivon low rate (data not shown).  The low rate of the Merivon treatment had significantly higher yield than the untreated check or the Actigard treatment.   The latter treatment had the lowest yield of any treatment, significantly less than any treatment except the untreated control.  

Lessons to be learned from the 2012 trial includes:

  • The two organic treatments, Saf-T-Side and Nordox, had significantly lower powdery mildew levels than the untreated check and not significantly different than the Actigard or Pristine/Rally treatment. 
  • Actigard, if used all season long, may reduce marketable yield and is not an effective powdery mildew product. 
  • As noted in my previous blog, Pristine may not be an effective powdery mildew product in Indiana anymore.
  •  The treatments with 3 products in alternation and the two Merivon treatments managed powdery mildew well.

As always, please don’t hesitate to contact me with any questions or concerns.

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Figure 1:  Powdery mildew management on zucchini with organic products.   Treatments with a letter in common are not significantly different at the 5% level (LSD).  

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Slidex2v.JPG

Figure 2:  Management of powdery mildew of cantaloupe with organic and conventional products.  Treatments with a letter in common are not significantly different at the 5% level.  

December 20
Fungicides for Powdery Mildew of Pumpkin

The last two summers, I have had pretty good fungicide trials for powdery mildew of pumpkin.  Since all of the products trialed are now labeled, I thought it was time to share this information with vegetable growers of Indiana. 

First, a bit of background about this disease.  In Indiana, powdery mildew affects primarily pumpkin and cantaloupe.  The disease is easily recognized by the talc-like lesions on both sides of the leaf. (This article will help with diagnosis.) If left uncontrolled, the disease can cause loss of foliage, loss of yield and lower quality fruit. 

The fungus that causes powdery mildew, Podosphaera xanthii, does not require leaf wetness for infection of leaves, only high humidity. The optimum temperature for disease development is 68 to 81 F.  P. xanthii may survive in crop residue as a resilient fungal structure, but the disease is so easily windborne, that crop rotation is not always a practical control measure. 

Fortunately, commercial varieties of pumpkin and cantaloupe exist with partial resistance to powdery mildew.  Most growers, however, find it necessary to apply systemic fungicides to manage powdery mildew, even when using partially resistant varieties.  The two trials I describe below use a susceptible variety of pumpkin, Gold Challenger, to assure plenty of disease.

In 2014, all of the fungicides used resulted in significantly less powdery mildew than the untreated check (Figure 1).  Fontelis alternated with Bravo weather Stik and Vivando used alone did not control powdery mildew as well as any of the other fungicide treatments. The best fungicide treatments were Luna Experience alternated with Quintec, Vivando alternated with Merivon, Aprovia Top at 8.5 fl. oz per acre alternated with Quintec, Aprovia Top at 10.5 fl oz. per acre alternated with Quintec and Fontelis alternated with Quintec (no statistical difference between these treatments).  There were no statistically significant yield differences between fungicide treatments; however, the untreated check has significantly fewer pumpkins than any of the fungicide treatments. 

The primary lessons for the 2014 trial may be summarized as follows:

·         Untreated, powdery mildew may cause loss of yield in pumpkins, at least with susceptible varieties. 

·         While Bravo WS, common name chlorothalonil, is useful against a broad range of diseases as a preventative fungicide, since this product is not systemic, it is not a good rotational product for powdery mildew. 

In 2015, the untreated check had more powdery mildew than any other treatment except for Pristine (Figure 2). This may indicate that the powdery mildew fungus has developed resistance to the two active ingredients in Pristine: pyraclostrobin, FRAC group 11, and boscalid, FRAC group 7 (FRAC stands for Fungicide Resistance Action Committee.  Each FRAC group represents a different fungicide mode of action.)

The next step down for fungicide efficacy in the 2015 trial, was Torino (unalternated) which was had significantly better control than Pristine, but not as good as any of the other treatments. Quintec unalternated was better than Torino, but not as good as the two remaining alternations. 

The treatments that resulted in the least amount of powdery mildew in 2015 included either Torino or Merivon alternated with Quintec and Procure (Bravo WS was tank mixed with Merivon, Torino and Quintec).  There were no yield differences in 2015, however, there was some interesting differences in handle quality due to powdery mildew severity. At harvest, approximately 4 inches of the stem next to the fruit (the handle) were removed, weighed and dried for 48 hours at 110 F and weighted again. From this data the percent dry matter in the handles were calculated. There was no difference in percent dry matter in pumpkin handles between fungicide treatments (Figure 3). However, the untreated check had a lower dry matter percent than any of the fungicide treatments.  Presumably, the reason percent dry matter was less in the untreated check is that powdery mildew caused fewer carbohydrates (photosynthates) to be translocated from the leaves to the handles. 

The take home for the 2015 trial could be summarized as:

·         Pristine may not be an effective management tool for powdery mildew of cucurbits in Indiana anymore. 

·         The best fungicide treatments may be those that alternate fungicide modes of action such as the two in 2015 that utilize Torino or Merivon with Quintec and Procure.

·         Even if yield is not directly affected by powdery mildew, fruit or handle quality may be affected as observed in this study.

Although Quintec is not a systemic product, this product may become redistributed around the leaf by vapor action. This product, in a proper alternation with other products using a different FRAC code, has proven to be effective.  Merivon, a relatively new product with a novel mode of action, appears to be effective for powdery mildew plus it is labeled for other diseases as well.  Torino appears to be a good powdery mildew product. 

For experimental purposes, not all treatments described here alternate fungicides with different FRAC groups or MOA's.  However, growers should know the FRAC groups for each of their fungicides and plan on a fungicide alternation between FRAC groups. Such an alternation will help to reduce the chance of creating fungi with resistance to one or more FRAC groups.  Plus, as seen here, alternating between fungicide FRAC groups often results in better disease control.

For further information, contact the author or the Midwest Vegetable Production Guide for Commercial Growers (the 2016 version is now on-line). ​ 


Slide1.JPG
Figure 1:  Fungicide pumpkin powdery mildew trial conducted at the SW Purdue Ag Center in 2014.  Blue bars represent powdery mildew disease severity in percent.  Orange bars represent yield in numbers.  Bars of the same color with the same lettter are not significantly different (alpha=0.05, LSD).  


Slide1z.JPG

Figure 2:  Fungicide trial for pumpkin powderty mildew conducted in 2015 at the SW Purdue Ag Center.  Bar represent powdery mildew disease severity in AUDPC (Area Under the Disease Progress Curve).  Bars with a different letter are not signficantly different (alpha= 0.05 LSD).

Slide2v.JPG

Figure 3:  Fungicide trial for pumpkin powderty mildew conducted in 2015 at the SW Purdue Ag Center.  Bar represent dry matter percent in pumpkin handles.  Bars with a different letter are not signficantly different (alpha= 0.05 LSD).​

October 26
Cercospora Leaf Mold of Tomato

This disease does not typically affect Indiana tomatoes, instead preferring tomatoes grown in tropical and sub-tropical areas. Since Cercospora leaf mold was observed in two different areas of Indiana in the 2015 season, it makes sense for growers to become aware of this disease in case it returns to Indiana in 2016.

The two locations where Cercospora leaf mold was observed in Indiana in 2015 were 1) a homeowner garden in southern Indiana and 2) a high tunnel in central Indiana. The fungus that causes Cercospora leaf mold, Pseudocercospora fuligena, normally does not overwinter outside of tropical and subtropical areas.  It may be that a wind blew the fungus in from the south in 2015.

Symptoms of Cercospora leaf mold are similar to leaf mold caused by Passalora fulva. Both diseases cause chlorotic (yellow) lesions which are visible on the upper side of the leaf. The chlorotic area caused by Cercospora leaf mold is more of a mustard yellow than that caused by P. fulva leaf mold in which the lesions are more diffuse and a brighter yellow (Figures 1 and 3). On the underside of the leaf, P. fulva leaf mold causes an olive-green fuzz that is from the causal fungus growing on the leaf.  Cercospora leaf mold can be differentiated from P. fulva leaf mold because the former is caused by a black fungus that grows primarily on the underside of the leaf (Figures 2 and 4). Neither disease causes lesions on stems or fruit. 

The causal pathogen of leaf mold, P. fulva, will overwinter as crop debris in the soil. This disease is often observed in high tunnels where high humidity and crops of tomato after tomato favors the disease.  Cercospora leaf mold will hopefully die out this winter in our cold climate.  Both diseases may be managed by sanitation.  Clean out high tunnel tomatoes between crops. A floor covering that prevents infected leaves from entering the soil will help lessen disease severity. In the field, practice crop rotation and till under the crop as soon as the last fruit is picked. 

Fungicides which control P. fulva leaf mold should help to lessen disease severity in Cercospora leaf mold.  The Midwest Vegetable Production Guide for Commercial Growers 2016 (coming January 2016) will help growers to choose a fungicide for P. fulva leaf mold. Always be sure to choose a fungicide labeled for greenhouse use if necessary. And always read the label. 

 ​clm fig 1.jpg

Figure 1: Cercospora leaf mold symptoms on the upper leaf surface.   Note distinct chlorotic lesions.  

clm 2.JPG

Figure 2: Underside of tomato leaf with Cercospora leaf mold. Note dark fungal growth.  

lm 1.JPG
Figure 3:  Lesions of leaf mold caused by P. fulva on tomato.  Note indistinct chlorosis.  

lm 2.JPG
Figure 4:  Underside of leaf with symptoms of leaf mold caused by P. fulva. Note olive-green fuzz of fungal growth.  



October 26
Symptoms of Anthracnose of Watermelon on Fruit

Late in the 2015 season, I observed some unusual symptoms of anthracnose on watermelon fruit. I wanted to discuss these symptoms, but first a little background of cucurbits. An extension bulletin on this subject may be found here.

Anthracnose of cucurbits, caused by Colletotrichum orbiculare, is responsible for lesions on leaves, stems and fruit. Crops affected include cucumbers and cantaloupe, however, watermelon is the host most often affected in Indiana. Although lesions on leaves and stems can cause significant loss, it is the lesions on fruit that cause direct yield losses. 

Lesions on watermelon fruit tend to be close to the ground where the fruit tends to stay wet. These lesions are typically round, sunken and orange to salmon colored (See figure 1). 

However, the lesions I observed toward the end of the 2015 season differed from the typical. Instead of regular round lesions, the symptoms I observed on the bottom of affected watermelon were cracked areas that at first glance appeared to be a wounds (Figure 2). Closer inspection, however, revealed the fungus C. orbiculare and lab isolations yielded the same fungus.  In addition, I was able to find foliar symptoms of anthracnose when I went to the affected field. While it is possible that secondary fungi infected and enlarged the anthracnose lesions, C. orbiculare caused the original infections.

Inspect fruit for lesions and, if necessary, have the lesions officially diagnosed. Only when the cause of the symptoms are understood will it be possible to manage the problem properly. ​ 

ant fig 1.JPG 

Figure 1:  Anthracnose on watermelon  fruit, caused by Colletotrichum orbiculare, is typically round and sunken.  

ant sm fig 2.JPG

Figure 2:  The long, cracked lesions on the watermelon shown above are anthracnose, althougt they are atypcial of this disease.  


August 30
Roguing as a Tool to Manage Phytophthora Blight of Pumpkin

When used as a verb, to rogue means to get rid of items that don’t conform to a certain standard.   In plant pathology, the word rogue is used to describe a technique whereby diseased plants are removed or rogued to slow the spread of disease.   I’d like to describe the practice as it might be used to manage Phytophthora blight of pumpkins. 

The practice works like this:  Under conducive conditions, Phytophthora blight spreads quickly from leaf to leaf and from plant to plant.  From a single diseased pumpkin plant, an entire field can become infected.  But what if one could rogue the few symptomatic plants at an early stage in the disease epidemic?  Would this slow the spread of Phytophthora blight?

In theory, yes.  If one were able to rogue all of the diseased plants in a field, the disease could be slowed.  It would be similar to sending sick children home from a classroom;  the disease should progress at a slower rate with sick children removed than if they had stayed and infected more children.  However, in practice there are a few complicating factors.  Read below for more details. 

Pumpkin plants may become infected with the organism that causes Phytophthora blight either by coming into direct contact with soil which harbors the causal organism or from spores that are spread from diseased plants.  The practice of roguing is designed to slow secondary or plant to plant spread of the disease. Phytophthora blight that is caused by direct contact with the soil will remain unaffected by roguing.  Therefore, roguing diseased plants will not stop new infections from soil borne fungi, however, this practice should slow the secondary or plant to plant-spread of Phytophthora blight.

Another complicating factor—It is almost impossible to completely eliminate all diseased plants.  The reason is that pumpkin plants with Phytophthora blight do not show symptoms immediately.  There is a period of 3 to 5 days between when the pumpkin plant is infected to when  symptoms become visible (in plant pathology, this is known as the latent period).  So, if one were to rogue all symptomatic plants, almost certainly some of the adjacent plants are infected but not showing symptoms yet.  The best solution to this problem is to rogue some of the healthy plants along with the diseased ones. Or, as they taught us in graduate school, rogue till it hurts. 

If one must remove apparently healthy as well as diseased plants when roguing, how many healthy plants must be rogued?  Unfortunately, there is no mathematical formula for estimating how many healthy plants to rogue.  However, let's assume that a rain storm accompanied by strong winds can blow splashed spores 10 to 15 feet.  If most pumpkin plants are on 6 foot centers, then one should remove about two rows of apparently healthy plants in addition to the diseased plants.  Each grower will have to estimate the amount of healthy plants to rogue based in his or her own circumstances. 

Roguing for disease management is most likely to be successful if attempted early in the disease epidemic.   Let's imagine that a few pumpkin plants are observed with Phytophthora blight in a low area of the field.  The decision to rogue is made.   The diseased plants are cultivated under as well as 2 or 3 rows of healthy plants beyond the plants with symptoms.  The cultivation equipment is cleaned off to prevent soil from the diseased field from being carried to a different field.  Such a situation is shown below in Figure 1.  While success is not guaranteed, roguing has the potential to slow disease spread.

A situation where roguing is less likely to be successful is one where much secondary spread has already taken place.  If a relatively large area of the pumpkin field already has symptoms of Phytophthora blight, the disease may have spread beyond where roguing may slow disease spread.  If the field has a long history of Phytophthora blight over most of the field, roguing may not help. 

In deciding whether to conduct a roguing operation to manage Phytophthora blight of pumpkin, it may help to know whether secondary spread of this disease has occurred.  Initial or primary spread of Phytophthora blight usually occurs in low areas of the field.  Since initial outbreaks of Phytophthora blight are likely to come from fungi that have survived in the soil, the first plants to be affected often have lesions where the plant has the most contact with the soil, at the very base of the plant where the main stem meets the soil. (Mature fruit which comes into contact with the soil may also have symptoms of Phytophthora blight. By the time mature fruit are present and symptomatic, however, secondary spread is likely to have occurred.)  Secondary spread, that is, disease that has occurred as a result of the splash of spores from a plant initially infected to the leaves and stem of healthy plants often occurs on the leaves, petioles or branches of the pumpkin plant. 

There are other circumstances where roguing may be used as a disease management tool.  Each circumstance, however, must be considered on its own merits.  Please let me know if you have any thoughts or questions. ​ 

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Figure 1:  In a field of pumpkins with Phytophthora blight, a portion of the field with symptomatic vines have been plowed down or rogued, to slow the spread of the disease.  


August 25
Northern Corn Leaf Blight

The relatively cool weather Indiana has experienced this summer may be responsible for more observations of northern corn leaf blight (NCLB) on sweet corn than normal.  The primary symptom is the cigar shaped lesion that ranges from 1 to 7 inches in length (see Figure 1). The lesions may range from tan to gray in color. Under conditions of high humidity, olive-green fungal spores may be produced on the lesion surface. Symptoms of NCLB are frequently observed late in the season when days become cooler. Yield losses are possible if lesions reach the ear leaf or higher during the two weeks before or after tasseling. NCLB can be managed by a combination of crop rotation, fall tillage, resistant hybrids and fungicide applications. Crop rotation and fall tillage help to minimize crop residue that might harbor the fungus that causes NCLB. Choose hybrids resistant to NCLB when possible. When it is necessary to use hybrids without resistance and weather conditions have been conducive to disease, fungicide may be used to help reduce symptoms of NCLB. See the Midwest Vegetable Production Guide for Commercial Growers for recommendations.  Effective fungicides for NCLB include Headline, Headline AMP and Quilt XCEL.  Fungicides may be less effective if applied after tasselling.  

 ​fig 1 nclb sm.JPG
Figure 1:  Northern corn leaf blight causes a cigar shaped lesion on the leaves of sweet corn.  

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 About this blog

 
 
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Dan Egel is an extension plant pathologist with Purdue University who works with vegetable growers across the state of Indiana. This blog will highlight recent disease issues, management options, meeting dates and new publications relevant to vegetable growers. Dan is located just north of Vincennes at the Southwest Purdue Agricultural Center.

 

Contact Information

Dan Egel
Southwest Purdue Agricultural Program
4369 N. Purdue Road
Vincennes, IN 47591
Phone: 812-886-0198

 www.watermelondr.info

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