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
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
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
Procure/Torino/Rally, Quinec/Torino/Rally and two rates of Merivon all had
significantly less powdery mildew than the organic treatments or the untreated
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
- 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
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).
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.
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
The primary lessons for the 2014 trial may be summarized as
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
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
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
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).
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).
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).
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.
Figure 1: Cercospora leaf mold symptoms on the upper leaf surface. Note distinct chlorotic lesions.
Figure 2: Underside of tomato leaf with Cercospora leaf mold. Note dark fungal growth.
Figure 3: Lesions of leaf mold caused by P. fulva on tomato. Note indistinct chlorosis.
Figure 4: Underside of leaf with symptoms of leaf mold caused by P. fulva. Note olive-green fuzz of fungal growth.
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.
Figure 1: Anthracnose on watermelon fruit, caused by Colletotrichum orbiculare, is typically round and sunken.
Figure 2: The long, cracked lesions on the watermelon shown above are anthracnose, althougt they are atypcial of this disease.
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
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
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
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
of the field, roguing may not help.
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.
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.
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.
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.
Figure 1: Northern corn leaf blight causes a cigar shaped lesion on the leaves of sweet corn.
I have observed this disease in several pumpkin fields this year. It is not clear to me why this disease seems to be more widespread compared to recent seasons. However, it makes sense to review Plectosporium blight here.
Plectosporium blight is usually not a serious disease. The occurence of this disease is usually sporadic. However, when it occurs, it can cause yield loss if left uncontrolled. Older literature may list this disease as
Microdochium blight. Plectosporium blight can be recognized from the light tan
lesions on stems and leaf petioles. Lesions may also occur on the fruit, although these symptoms are less common. Yield loss is most often caused by lesions on the stem adjacent to the fruit-the pumpkin handle. Yellow
squash and zucchini squash are also affected. Lesions are often individually spindle shaped. When these lesions occur in large numbers they can give a light gray or white appearance to the foilage. This disease
may be managed through a combination of cultural and fungicide treatments. Crop
rotations of 3-4 years and fall-tillage will help keep the crop residue to a
minimum. A regular contact fungicide program will also help to keep
Plectosporium blight in check.
Figure 1: The handle of this pumpkin has lesions of Plectosporium blight which may ruin the marketablity of the fruit.
Figure 2: Plectosporium blight may cause the stems and petioles of pumpkin plants to appear white or light brown when numerous spindle shaped lesions coalesce.
Downy Mildew has been confirmed on jack-o-lantern pumpkins in Daviess and Jasper Counties. This is the first time that this disease has been confirmed on pumpkins in Indiana in the 2015 season. There are unconfiirmed (but reliable) reports of downy mildew on pumpkins in Parke and Washington County. This disease has also been observed on butternut squash in Knox County. Read more about this disease here.
Late blight has been reported on processing tomatoes in Cass County Indiana. This is a late blight update from when this disease was reported on potatoes and tomatoes in LaGrange County Indiana. The latter outbreak and some disease management tips are reported in the Vegetable Crops Hotline here.
Protecting vegetable crops from foliar disease involves
many factors. Crop rotation and fall
tillage will help to lessen disease severity. Choosing a resistant or partially resistant
variety can lower the amount of disease. Purchasing seed that has been tested for seed
borne disease is also an important factor.
Most growers, however, find it is also necessary to apply fungicides to
manage foliar diseases. This article
will discuss when such applications are productive-and one case where they may
Foliar fungicides are most effective when applied before
infection of a plant disease takes place or early in the disease epidemic. That is, it is best to apply fungicides
before one observes disease and at regular intervals. Fungicides are designed to protect healthy
foliage from disease. Applications of
fungicides will not change plant tissue that has been turned brown (necrotic)
from disease into green healthy tissue.
Figure 1 shows a watermelon field with severe symptoms of
anthracnose. Large areas of the field
appear brown due to anthracnose infection.
This situation may have resulted from missed fungicide applications,
lack of crop rotation or weather conditions very conducive to disease. In any case, the common reaction of a grower
upon viewing such a field is to apply a fungicide. Many times the reaction is
to reach for an expensive cure. Again, no fungicide will turn brown plants
into green plants.
When the decision to apply fungicides to a vegetable field
with severe foliar symptoms is made, the grower should make such an application
with the aim of protecting healthy plants.
Is there a large enough area of healthy plants to justify a fungicide
It may be argued that some systemic fungicides have what
may be referred to as ‘kickback action’. That is, the fungicide is designed to
move in the plant and stop fungi inside diseased tissue. Won’t such a fungicide bring plants back from
a diseased situation?
The successful application of many systemic fungicides with
‘kickback action’ will stop or slow fungi inside plant tissue. One should think about such action as
inhibiting fungi at the edge of a plant lesion before it can expand into
healthy tissue. Even if fungi are
stopped inside necrotic tissue, the tissue will remain brown. Thus, fungicides should be applied to protect
green tissue from disease.
When applications of fungicides are made to fields such as
in Figure 1, one should also remember that the disease has almost certainly
spread beyond the dead plants. Since it
takes 7 to 10 days for diseased plants to show symptoms, more of the plants are
diseased than what are observed in Figure 1.
An argument can be made that some crops may be able to
regrow healthy leaves, branches and fruit from diseased plants if kept healthy
with fungicides. The decision to apply
fungicides to allow regrowth of healthy plant tissue will depend of the cost
and efficacy of the fungicide(s), the weather (dry weather will be more likely
to favor healthy growth), market value of the produce and stage of crop.
Applications of a fungicide to non-productive plants may be
made as a way to reduce disease inoculum that may otherwise spread to heathy
plants. If one is realistic about why
such a fungicide is applied, then this type of application can be
justified if the cost is right. Another management tool is to
remove or rogue diseased plants to stop disease spread. But this is a topic for another time.
My intent is not to be pessimistic about vegetable
production or fungicide use. Instead, I
would like to point out the importance of timely use of fungicides, especially
early in the season. In contrast,
fungicides applied late in the season to a crop already very much affected by
foliar disease may not be useful.
Vegetable growers should be realistic about applications of fungicides
to late season crops with significant amounts of disease.
There are large areas of this watermelon field that appear brown from the disease anthracnose. No amount of fungicide will cause these areas to turn green. Growers should assess the likelihood that fungicides will be effective before spending large amounts of money on an application.