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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. 

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

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

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

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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. ​ 

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

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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.  

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Figure 1:  Northern corn leaf blight causes a cigar shaped lesion on the leaves of sweet corn.  

August 24
Plectosporium Blight

 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.

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Figure 1:  The handle of this pumpkin has lesions of Plectosporium blight which may ruin the marketablity of the fruit.  

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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.

August 24
Downy Mildew of Cucurbit Update

​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.​   

August 24
Late Blight on Tomato Update

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​.  

August 16
To Spray or Not to Spray

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 not be. 

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 application? 

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. ​ 

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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.  

July 23
Botrytis Gray Mold of Tomato

Gray mold of tomato is one of the more common diseases of greenhouse-produced tomatoes. Although it is often a minor problem, if left unchecked, gray mold can casue yield loss.

Gray mold, or more properly, Botrytis gray mold, often causes a light gray or brown necrotic lesion on leafs. The lesions on leafs are sometimes wedge shaped on the margin of the leaf.  Stem lesions are a similar color and may encircle the stem, causing the death of the upper portion of the stem.  Occasionally, gray mold may cause the rot of tomato fruit. Whether on leafs, stems or fruit, the gray fungal sporulation is often easily seen, thus the name. It is a rare symptom, but when fungal spores land on tomato fruit that is wet, the spores may germinate, causing a symptom known as a ghost spot. 

Botrytis gray mold can cause disease on many different host plants, enabling the fungus to easily survive and disperse between tomato crops.  Host crops include flowers such as geraniums as well as other vegetables such as green beans. The disease is favored by relatively cool temperatures and high humidity which explains why the disease is often observed in greenhouses. 

Any cultural practice that lessens humidity such as pruning, will lessen the severity of gray mold. Since gray mold favors older plant tissue, pruning old leafs should reduce susceptible plant tissue.  As a general rule, indeterminate tomatoes should be left with no more than eighteen to twenty fully mature leafs after pruning. Determinant (staked) tomatoes are often pruned until the first flower cluster, improving airflow and encouraging larger fruit.  Another practice that may reduce airflow is spacing plants too close together.

Practicing crop rotation may reduce the amount of the gray mold fungus that survives in a greenhouse. If crop rotation is not possible, remove as much of the crop as possible far away from production when the season is complete. Clean and sanitize the greenhouse between tomato crops. Use of a ground covering between rows of tomatoes may help to reduce the amount of crop residue that becomes incorporated in the soil. 

Several fungicides that may help to manage gray mold are listed in the Midwest Vegetable Production Guide. Products that should be effective against gray mold and are allowed for greenhouse/high tunnel use in Indiana include:  Botran, Fontelis, Scala and Switch. Products that contain the active ingredient mancozeb may be less effective than those listed above, but these products may be less expensive and are readily available. Organic producers should look for products that contain formulations of copper. ​​ 

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Figure 1:  Leaf lesions of Botrytis gray mold are often a light gray or brown color and the sporulation of the causal fungus can be seen on the leaf margin.  

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Figure 2: Although less common than leaf or stem lesions, ghost spots may also be caused by gray mold.

July 22
Powdery Mildew of Tomato

This disease has been reported near West Lafayette and in Wanatah Indiana.  Powdery mildew of tomato can be recognized by the white fungal colonies on both leaf surfaces.  Occasionally, stems may also be infected.  Severely affected tomato plants may have leafs that turn chlorotic and necrotic.  Fruit will not be directly affected. 

The causal organism has been identified as Pseudoidium neolycopersici, formerly Oidium neolycopersici.  This fungus may survive as resting structures on host material.  The spores are easily wind dispersed to additional tomato plants.  Development of this disease is favored by temperatures below 86F.  As with most powdery mildew diseases, high humidity allow the disease to develop; leaf wetness is not necessary.  Since high humidity favors powdery mildew of tomato, greenhouse environments often favor the disease. 

Reports of powdery mildew on tomato are not common in Indiana.  There is no data on yield loss from this disease on tomato.  Nevertheless, if this disease is present, management options should be considered.  Several systemic fungicides are listed as possible options in the Midwest Vegetable Production Guide.  It might be useful to choose a product that is also labeled on early blight and Septoria leaf blight.  An example would be Fontelis, which is labeled for field or greenhouse tomatoes. ​ 

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Figure 1:  Powdery mildew of tomato is easily recognized by the white fungal sporulation on both sides of the leaf as well on the stem. (Photo:  Natasha Cerruti).  

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 About this blog


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

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