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June 23
Phytophthora Fruit Rot of Watermelon

Most growers first notice this disease when large soft areas develop on mature watermelon fruit. These soft, rotten lesions can be several inches across and are often covered with a white mold.  The lesions usually form first on the bottom of the fruit, close to where the fruit comes into contact with the soil.  Further development of the disease often results in lesions on the top of the fruit as well. 

Conditions that favor Phytophthora Fruit Rot include warm, rainy weather such as have occurred over much of Indiana during the past few weeks.  Water that stands in pools also favors severe disease symptoms. Overhead irrigation may help the disease to spread. Phytophthora Fruit Rot can spread rapidly when conditions are favorable. 

The organism that causes Phytophthora Fruit Rot of watermelon is Phytophthora capsici. This organism is more closely related to algae than to fungi. Therefore, P. capsici is sometimes referred to as a fungus-like organism.  The close relationship of P. capsici to algae helps explain why this disease is favored by water; one of the life stages is a spore that is motile is water. Perhaps because of the taxonomic difference between P. capsici and most fungi, the fungicides that are most affective against P. capsici are less affective against most fungal diseases. 

To control this disease, choose fields that are well drained, preferably fields that do not have a history of the disease. Watermelon fields with plastic mulch and raised beds may have less disease severity. 

Fungicides, if used, are best applied before the disease appears. This is because by the time a few lesions appear, many more lesions are present, but are too small to see. For the most part, growers will want to wait until fruit are present to apply fungicides. Applications to small fruit may include Forum® or one of the products with phosphorous acid as an active ingredient (e.g., Agri-Phos®, Phostrol®, Kphite®).  However, during conducive conditions (such as rainy weather on fields with a history of the disease) Presidio® can be alternated with Revus®.  A newer product, Zampro®, has also proven effective. 

Be careful to alternate fungicide modes of action, or FRAC codes. For example, Revus®and Forum® both have 40 FRAC codes and should not be alternated.  Zampro® has two FRAC codes, 40 and 45, and therefore should not be alternated with Revus® or Forum®

Phytophthora Fruit Rot of watermelon can be a serious disease.  Prepare for this disease before it appears in a field near you. 

 

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Figure 1:  Phytophthora fruit rot of watermelon causes large, soft lesions on the surface of watermelon fruit.  Under moist condiitions, the lesions may be covered with a white mold.  

June 06
Black Rot of Crucifers

Cabbage is the crop most often affected by black rot, however, other crucifers such as broccoli, cauliflower, mustard, Kohlrabi or Brussel sprouts may be affected.  The first symptom one is likely to notice is a ‘V’ shaped lesion on the margin of the leaf (Figure 1).  However, the symptom on Brussel sprouts observed recently are irregular, jagged lesions on leafs (Figure 2).  The plants represented in Figures 1 and 2 are different varieties of Brussel sprouts. The differences may be due to differences in susceptibility of the two cultivars or the cultivar in Figure 2 may have been infected at an earlier age than the one in Figure 1. Figure 3 shows two severely affected plants next to a relatively healthy plant. 

Black rot is most severe in wet, warm weather. The emergence of this disease during a rather cold spring may mean that the disease started in a greenhouse situation. 

The bacterium that causes black rot, Xanthomonas campestris, survives in crop residue.  Thus, crop rotations that avoid crucifers should lessen the severity of the disease.  Sanitation in the greenhouse should help to lessen the amount of the bacterium that can cause more disease.  The causal bacterium may also be transmitted through seeds, therefore, every effort should be made to plant seed that has been tested and found free of the bacterium.  Inspect transplants for symptoms before planting.  Avoid practices which add to free water to plant surfaces.  Products which contain copper as an active ingredient may help to lessen the spread of the disease. However, copper products may also cause lesions on leafs under some circumstances.  Although there may be varietal differences in susceptibility, compete levels of resistance are not available in commercial cultivars. More information about general pest management can be found in the Midwest Vegetable Production Guide for Commercial Growers, 2015Midwest Vegetable Production Guide for Commercial Growers, 2015.

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Figure 1:  Typical symptoms of black rot include 'V' shaped lesions on the margins of leaf.

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Figure 2: The angular lesions on this Brussel sprouts leaf are not typical of black rot.


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Figure 3:  The two Brussel sprouts plants on the left are severely affected by black rot.  

June 06
Pith Necrosis of Tomato

This disease has been reported in two different greenhouse situations.  Although the disease is not usually economically important, a brief review of the disease is offered here to help tomato growers differentiate pith necrosis from more important problems. 

Tomato pith necrosis causes dark brown streaks on tomato stems and leaf petioles (Figure 1).  Often stems may appear twisted and distorted. When cut open, the stem may appear discolored and chambered (Figure 2). Eventually, the affected plant may become stunted and wilt. Tomato pith necrosis is usually found in greenhouses or high tunnels.  

Because the plant has a discoloration in the stem, it is sometimes confused with bacterial canker, a much more serious disease. A comparison of the two diseases can be found here

It is not clear how pith necrosis spreads or enters the tomato plant, but it is probably best to remove affected plants and avoid using pruning equipment on diseased plants. When removing diseased plants, it is always best to leave as little of the plant behind as possible. A landscape cloth covering can help to keep crop residue out of the soil.

Perhaps since pith necrosis is not economically important and does not appear to spread quickly, not much is listed for the management guidelines. To manage tomato pith necrosis, avoid low night temperatures and excessive nitrogen levels; reduce high humidity in the greenhouse or high tunnel. General pest management guidelines can be found in the Midwest Vegetable Production Guide for Commercial Growers 2015

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Figure 1:  Pith necrosis of tomato may result in dark, necrotic streaks on stems.  

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Figure 2:  Pith necrosis of tomato may cause internal discoloration and a chambered internal stem.   


May 27
Fusarium Wilt of Watermelon

Fusarium wilt of watermelon is often observed on about Memorial Day in Indiana.  This is because at this date, watermelon vines are usually 6 to 10-inches long, the stage where Fusarium wilt often shows up.  This year, most growers are behind in their planning due to the cold, wet weather.  The Fusarium wilt I observed this week were on vines that had been under low row covers.  In addition, last week I also observed Fusarium wilt of watermelon still in transplant trays.

Symptoms of Fusarium wilt of watermelon include wilting and stunting. Often one leaf or vine may be wilted leaving the rest of the plant apparently healthy (Figure 1). Seedlings that are affected are often not big enough to show such one-sided symptoms (Figure 2).  While the roots may be white and healthy, the interior of the stem may have a brown discoloration. Relatively cool weather seems to favor the fungus over the watermelon seedling. Thus, cool weather in May and early June sometimes leads to a higher incidence of Fusarium wilt. In addition, this disease is often found in well-drained areas. Regardless of how many watermelon plants appear to be dying at the moment, remember that this disease has not been shown to spread from plant to plant in the field. Typically, no more than 10 percent of plants will be killed due to Fusarium wilt of watermelon. In addition, once the weather turns warmer, the watermelon plant will begin to outgrow the fungus. It is possible to spread the fungus that causes Fusarium wilt of watermelon through soil that remains attached to cultivation equipment between fields. In order to minimize the spread of the fungus between fields, clean off soil between fields with high-pressure water. It may not be practical to disinfest the equipment between fields; however, one might spray a solution of a quaternary ammonia solution (Greenshield or Physan 20) or 10 percent bleach on the tools. Growers will want to make every effort to keep infected seedlings out of commercial fields. Closely inspect transplants before planting them. Fusarium wilt can be transmitted on seed. Previously used trays may harbor the fungus that causes Fusarium wilt. Although transplant trays can be disinfested, it can be very difficult to clean and disinfest trays sufficiently to eliminate the possibility of Fusarium wilt.

Use long crop rotations of at least 5 to 6 years between watermelon crops since the fungus that causes Fusarium wilt survives for several years without a host planted in the field. The fungus that causes Fusarium wilt of watermelon is very specific to watermelon and will affect no other crop plants. Likewise, Fusarium of other crop plants like tomato and cabbage will not affect watermelon. While no variety is completely resistant to Fusarium wilt of watermelon, there are differences in susceptibility. The Midwest Vegetable Production Guide for Commercial Growers 2015 has more information about Fusarium wilt of watermelon including a soil applied fungicide.  This extension bulletin also has information about Fusarium wilt of watermelon. Feel free to call me with comments or questions. ​ 

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Figure 1:  Fusarium wilt of watermelon often causes one vine to wilt while the remainder of the plant appears healthy.

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Figure 2:  Fusarium wilt may also occur on watermelon seedlings in transplant trays.  

May 17
Watermelon Disease Update

Most watermelon growers are in the process of placing transplants in the field.  I have received several commercial samples of transplants still in trays prior to out-planting.  The two diseases I have observed so far are gummy stem blight and bacterial fruit blotch.  Below, I discuss these two diseases as well as management options. 

Gummy stem blight on transplant seedlings may be recognized by the watersoaked area of the stem (botanical term:  hypocotyl) as shown in Figure 1.  The watersoaked area may eventually turn brown and woody.  A closer look at the woody area may reveal the small, dark fungal structures of the gummy stem blight fungus (Figure 2).  The true leaves of watermelon transplants may also be affected. 

The fungus that causes gummy stem blight (Didymella bryoniae) may survive in crop debris, thus overwintering in the field from year to year.  This fungus may also survive in seed.  It is also possible for the fungus to survive for short periods in greenhouse production facilities. 

Crop rotations with non-cucurbit crops for 3 years will help to lessen disease severity.  Preventive fungicide applications may be scheduled with MELCAST, a weather-based disease forecasting system.  Contact fungicides such as chlorothalonil (e.g., Bravo, Echo, Equus, Initiate) or mancozeb (e.g., Dithane, Manzate, Penncozeb) should be alternated with systemic products such as Luna Experience, Switch, Inspire Super or tebuconazole (e.g., Monsoon).  Not all of these fungicides are labeled for other fungal diseases such as anthracnose.  Remember to alternate modes of action by using the FRAC codes of the fungicides.  See the Midwest Vegetable Production Guide for more information. 

The symptoms of bacterial fruit blotch (BFB) can be difficult to recognize on foliage.   Leaf lesions may be angular and appear to run along the vein (Figure 3).  The lesions may appear watersoaked, especially when viewed on the underside of the leaf.  Leaf symptoms of BFB are easily confused with angular leaf spot, a disease that is not often economically important.  A laboratory analysis may be required to distinguish these two diseases.  The relatively large, oily lesions on fruit are easier to recognize (Figure 4).

In contrast with gummy stem blight described above, the bacteria that causes bacterial fruit blotch (Acidovorax avenae subsp. citrulli) does not readily survive in crop residue.  The bacterium is known to survive in seed.   It is possible that the bacterium may survive in greenhouse production facilities for short periods. 

Once BFB is detected in the field, applications of a copper product tank mixed with a  mancozeb product may help to lessen disease severity.  Whether BFB is detected in a watermelon field or not, updated recommendations are to apply copper 2 weeks before first female bloom, at first female bloom and 2 weeks after first female bloom. Additionally, application of the product Actigard at 2 of the 3 copper application times listed above is recommended. More information about these new recommendations may be found here​ or in the Midwest Vegetable Production Guide. 

Be sure to inspect seedlings for signs or symptoms of disease.  Avoid planting transplants that may be diseased.  


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Figure 1:  This watermelon transplant has a water soaked area just under the seed leaves, a typical symptoms of gummy stem blight.

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Figure 2:  A more advanced symptom of gummy stem blight is the light brown woody appearing area of the stem near the seed leaves.  The dark structures of the fungus that causes gummy stem blight may be observed with a 10X hand lens. 

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Figure 3:  Lesions of bacterial fruit blotch of watermelon on transplants may include angular lesions that may appear water soaked. ​

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Figure 4:  Mature watermelon fruit may have large, dark, irregular lesions due to bacterial fruit blotch. ​ 

April 29
Fusarium crown and root rot of tomato

Symptoms of this disease include tomato plants with lower leaves that become yellow (chlorotic) and die; plants that begin to wilt; a lesion on the lower stem at ground level (Figure 1 and 2).  If tomato plants are removed from the soil and carefully split open from the ground level, a discoloration of the vascular tissue can be observed (Figure 3).  It is important to note that this discoloration does not extend up the stem more than 6 to 8 inches.  If the discoloration extends up into the plant canopy, the disease maybe Fusarium wilt of tomato.  Although growers may observe multiple plants begin to die of this disease over a period of days or even weeks, the fungus does not splash from plant to plant.  Therefore, there should be no plant-to-plant spread in the high tunnel.

Temperatures from 68 to 72 degrees F favor Fusarium crown rot and may explain why I observed this disease last week when the weather was relatively cool.   I often observe Fusarium crown rot in high tunnel or greenhouse situations where the tomato plants are grown in the ground.  This is because the causal fungus, Fusarium oxysporum f.sp. radicis-lycopersici, survives very well in the soil in the absence of the host. 

Crop rotations that do not include tomatoes or other solanaceous crops will help to lower the amount of fungal spores in the soil. However, since the causal fungus survives for years without a host, crop rotation is not a complete solution.  I also realize that many growers who produce tomatoes in high tunnels do not feel it is economically practical to rotate to another crop.  To such growers, I would point to this article about how to minimize diseases in high tunnels. 

Growers who plant tomatoes in bags or pots in a high tunnel instead of in the soil should avoid Fusarium crown rot since the fungus survives in the soil. 

Check with your seed representative or seed catalog for tomato varieties with resistance to Fusarium crown rot.  Most tomato varieties with resistance to Fusarium crown rot are indeterminate.  (In contrast, there are many varieties with host resistance to Fusarium wilt.)  It is possible to graft your favorite tomato variety as a scion onto a rootstock variety with resistance.  This table will help one select tomato rootstocks with resistance to Fusarium crown rot and other diseases.  Some tomato seed companies will sell grafted tomatoes. 

There are no fungicides to control Fusarium crown rot.  Most fungicides are for foliar use; I know of no fungicides that may be sprayed on the top of the soil.  Read the label carefully and contact me if you have questions. 

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Figure1:  The tomato plants shown here are stunted, wilted and the lower leafs are dying due to Fusarium crown and root rot.  

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Figure 2:  The lesion at the base of the stem is typical of Fusarium crown and root rot of tomato.  

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Figure 3:  Vascular tissues are discolored in this tomato stem as a result of Fusarium crown and root rot.  Note that discoloration only goes a few inches up the stem.  Tomatoes with Fusarium wilt have a similar discoloration that goes up into the canopy of the plant.  


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April 18
Fertility for the Southwest Purdue Agricultural Center Tomato Trial

 Last year at the Southwest Purdue Agricultural Center (SWPAC) we conducted a tomato high tunnel trial described here.  In this article, I would like to talk about the trial we will conduct in 2015, a repeat of the 2014 trial.  In particular, I would like to talk about what we have done for fertility. 

Before deciding on a fertility scheme, it is critical to conduct a soil test each year.  Our soil test from November 2014 showed that our high tunnels were low in sulfur, boron and moderately low in zinc.  In fact, plant tissue tests conducted during the 2014 season were low for both sulfur and boron.   As a result of these tissue tests, we added a 10% liquid boron product and ammonium thiosulfate (7%) to the fertigation during the 2014 season. However, the next set of tissue tests carried out during the 2014 season also came back low in these two elements.  It wasn’t until the end of the 2014 season that we observed levels of boron and sulfur close to normal.  It may be that when tomato plants are growing very quickly, it is difficult to add sufficient nutrients to keep up with demand. (I should add that our yields of tomatoes were over 140,000 lbs on a per acre basis for the 2014 season.  The low boron and sulfur tissue tests didn’t seem to hurt our yields too much.  However, if we hadn’t monitored by tissue tests and added boron and sulfur, the yield may have been affected.) Since we had trouble keeping up with boron and sulfur levels in 2014, this year we decided to add 2.5 lbs per acre of zinc sulfur (10/7%) and 1.5 lbs. per acre boron (14.3%) pre-plant broadcast (see update below).  We also added 200 lb per acre pelletized lime. 

During the season we will add nutrients at every irrigation (fertigation).  We transplanted on April 2, 2015, adding a cup of 20-20-20 liquid starter fertilizer per plant.  We started fertigating potassium nitrate (KNO3) on April 6.  We mix 2 oz. of KNO3 per gallon which is then applied at a ratio of 1:100 at each fertigation.  Each high tunnel has 5 rows 80 feet long that has drip tape and black plastic mulch.  We started out fertigating 20 gallons twice a day per high tunnel.  Five days later, we started fertigating 20 gallons 3 times a day. On April 21, we started giving the tomatoes 30 gal 3 times per day.  

We try to avoid fertigating with pre-mixed products such as a 20-20-20 through the drip.  Such products almost invariably add elements that are not needed.  In our case, for example, phosphate is not needed and would be added in most general mixes. Adding elements that are not needed may lead to a buildup of salts in the soil.  This is a particular problem where tomatoes are grown year after year in a greenhouse or high tunnel. 

Please return to this blog to hear about other developments in our high tunnel projects or other vegetable issues that I encounter during the season.  And feel free to contact me with questions or comments.  


Update June 5-We just recived our first foliar nutrient tests back.  In many of the samples, the boron was listed as 'excessive'.  We don't see any advserve reaction of the tomato plants yet, but it looks as if we over did the boron.  

April 15
Watermelon Fungicide application Schedule

Below I have written some hints on how to develop a watermelon fungicide application schedule.  When I presented this at the University of Kentucky this winter I promised to post this  So, I am late.  Remember, do not wait until symptoms of disease develop.  Keep a regular schedule.  Be sure to have an official diagnosis of any symptoms you are not sure of.  To help watermelon growers apply fungicides according to the weather, Dr. Rick Latin at Purdue Unviersity developed MELCAST, a weather-based disease-forecasing system.  

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Figure 1:  This diagram shows a possible fungicide schedule for watermelon.  Here, the contact fungicide with the active infredient chlorothalonil or mancozeb are used all season long.  Since both of these active ingredeints have the FRAC code M, there is no need to alternate to a different mode of action.  Most chlorothalonil products have a 12 hour re-entry interval (REI)l and a 0 day pre-harvest interval (PHI).  Most mancozeb products have a 48 hour REI and a 5 day PHI.  

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Figure 2:  In this scheme, contact fungicides such as used above are used for most of the season (red arrows), but relatively more expensive systemic fungicides are suggested from mid to late June.  Note that a contact fungicide is used in-between the use of the systemic fungicides.  It is important not to use fungicides with the same FRAC codes back to back.  I have tried to suggest that the systemics might be used at about lay-by.  That is, at the point where the vines are piled on top of the plastic mulch-just after the last vine turning.  

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Figure 3:  Here I suggest that the same systemic fungicide, Pristine, is used with an application of Bravo or some other contact fungides used in between.

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.Figure 4:  Here I suggest that two different systemic fungicides may be used back-to-back if they have different FRAC codes, that is if they have different modes of action.  

Some general comments.  Contact fungicides, such as chlorothalonil and mancozeb, are designed so that if applied to foliage in a manner to get good coverage the foliage is protected.  That is, the fungicides covers the foliage and inhibits spores that may be deposited on leaves.  Contact fugncides do not move into the plant.  


Systemic fungicides move into the plant.  But how far the system fungicides moves within the plant depends on the product.  Most do not move more than inch.  Almost all systemic fungicides move toward tip of the plant, not toward the base or roots.  Systemic fungicides may have some efficacy against exisiting disease, but it is still much more effective to apply all fungicides before the disease gets started.  

I have mentioned a few fungicides in particular here, but just as examples.  There may be other fungicides more effective for your purpose  See the Midwest Vegetable Production Guide for Commercial Growers​ for more information.



April 09
Cucurbit Production in Indiana: 1925 and 2013

In December 2014​, I described the ‘Yearbook of Agriculture, 1928”. In that blog, I wrote about processing tomato production in 1925 and 2013 (the ‘Yearbook of Agriculture, 1928’ lists data back to 1925).  Today, I would like to discuss cantaloupe and watermelon production. Unfortunately, yields posted in the “Yearbook” are in different units than in use today. However, I can compare acreage in 1925 and 2015. 

Cantaloupe production in Indiana in 2013 was at 2,100 acres. This compares to 4,820 acres in 1925. Part of the reason for the drop in acres might be that cantaloupe requires a lot of postharvest handling.  Buyers want cantaloupe, also known as muskmelon, to be washed and cooled. Food safety concerns require growers to invest in specialized equipment and wade through reams of regulations.

In 1925, Indiana was number 6 in the US in cantaloupe acreage, behind California and Arizona (of course) as well as Colorado with 7,900, Arkansas with 7,730 and Maryland with 5,570.  The 2013 USDA data, which lists Indiana as number 4, doesn’t even list Arkansas and lists Maryland with 630 acres and Colorado with 600.  It is interesting to note that total acreage for cantaloupe production has dropped from 93,260 in 1925 to 70,410 in 2013. Acreage appears to have consolidated in states like California, Arizona and Indiana.  My guess is that yields per acre have also gone up, but I can’t compare given the units used in 1925. 

I mentioned that production tracked in different units back then.  In 1925, Indiana produced 627,000 ‘crates’ of cantaloupes. I am not sure what a crate is; it may have been in crates of 36-45 cantaloupe.  In any case, an average price per crate in 1915 was $1.29.  If correct that means that cantaloupe were 2 to 3 cents per fruit! 

If one uses the above production data for 1925, then cantaloupe production value in Indiana was $808,830. Figures for 2013 give a value of $11,500,000. Of course, inflation accounts for much of the increase in value.  Whatever the reasons for the increase, the cantaloupe industry in Indiana is healthy. 

Watermelon acreage in Indiana went from 3,440 in 1925 to 7,400 in 2013. It seems that what was lost in cantaloupe acreage has been made up for in watermelon acres. Watermelon do not have the same post-harvest needs as cantaloupe.  In 1925 Indiana was number 8 in the US.  In 2013, Indiana was number 6. A couple of states that Indiana jumped ahead of in the intervening years are Missouri and Alabama with acreage at 12,200 and 10,030, respectively.​

April 03
Updates to the Midwest Vegetable Production Guide, 2015

While the hard copy of the Midwest Vegetable Production Guide for Commercial Growers 2015 (ID-56) has been available since early January, the on-line version is updated as needed. Below I outline the latest changes.

Page 40, Table 16.  Several insecticide products were added to the “Insecticide Labeling for Greenhouse Use” table. 

Page 100, Cucurbit chapter.  Luna Privilege® was removed from the lists of suggested products for Alternaria leaf blight control and gummy stem blight/black rot control. While Luna Privilege is labeled for these uses, it is not available yet.

Page 100, Cucurbit chapter.  The rates for Presidio® for downy mildew and Phytophthora blight control were modified by the manufacturer. 

Page 109, Product/Disease Rating for All Cucurbits.  Several products were deleted, added, or modified in the Product/Disease Ratings for All Cucurbits table.  These include Luna Experience®, Actigard, Revus ® and Presidio®.

Page 125, Fruiting Vegetable Chapter.  Ridomil Gold SL® was added to the list of recommended products for buckeye rot and Phytophthora blight control in tomato.

Page 125, Fruiting Vegetable Chapter.  Gavel 75DF® was added to the list of recommended products for leaf mold control in tomato. 

Page 130, Product/Disease Ratings for All Fruiting Vegetables.  Several products were deleted, added, or modified in the Product/Disease Ratings for All Fruiting Vegetables table.  These include Bravo®, Dithane®, Priaxor®, Quadris Top®, Fontelis® and Inspire Super®

Note that the index page http://mwveguide.org/ for the on-line version of the ID-56 details the change history.  If you have questions or comments about any of these changes or want a hardcopy of the changes, contact me at egel@purdue.edu or (812) 886-0198

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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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