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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).  

July 22
Downy Mildew of Watermelon

This disease has been observed on watermelon in Knox County.  The following article will discuss the symptoms, biology and management of downy mildew of cucurbits.

Symptoms-The symptoms of downy mildew vary depending on the host.  

·         On watermelon, the lesions start out as chlorotic (yellow) areas that become round and necrotic (brown/black) areas surrounded by a chlorotic halo.  Lesions may be limited by veins (Figure 1).  Note that leaf lesions of gummy stem blight may have dark fungal structures (pycnidia) present that are lacking with downy mildew.  Also, whereas gummy stem blight will affect stems and petioles, downy mildew will not. 

·         Pumpkin lesions also start out chlorotic and are often angular.  Eventually, the chlorotic lesions become necrotic.  Lesions may be more common along a vein.

·         Lesions on muskmelon often have poorly defined margins and are not as angular as described above for pumpkin. 

·         Cucumber lesions start out chlorotic and very angular, eventually becoming necrotic.

For all hosts, the bottom of the lesions becomes covered with a dark ‘fuzz’ under moist conditions.  The ‘’fuzzy’ appearance is due to the production of spores that are easily wind-borne.  Severely affected leafs become crumpled and brown and may appear scorched.  Downy mildew does not affect stems or fruit directly. 

Downy mildew of cucurbits is favored by temperatures of 59 to 68 F.  Relative humidity of 100% for 6 hours is sufficient to allow infection to take place on a leaf.  Once infection takes place, spores are produced on the underside of the leaf.  The spores are dislodged upon drying and easily become airborne. 

The fungus-like organism that causes downy mildew does not overwinter in Indiana; instead it overwinters on the Gulf coast or in cucurbit greenhouse production somewhere near the Canadian border.  Usually, this fungus-like organism blows into Indiana in late August or September.  This year’s entrance is unusually early. 

The fungus-like organism that causes downy mildew of cucurbits does not affect unrelated hosts such as soybean. 

There are several pathotypes of the fungus-like organism that cause downy mildew.  Each pathotype specializes in a different set of cucurbit hosts.  In most years, downy mildew shows up on cucumbers and cantaloupe first.  Since, this situation is a bit complicated and isn’t entirely understood, I would advise all cucurbit growers in Indiana to begin the management options described below even if it is only to closely scout one’s field. 

While crop rotation is always a good idea, this management technique will not work for downy mildew since the causal organism does not overwinter in the soil.  There are a few varieties that have resistance, but only for cucumbers.  Most growers find it necessary to apply fungicides to protect their cucurbit crops.  However, since the fungus-like organism that causes downy mildew is not closely related to the fungi which cause most other cucurbit diseases, the fungicides that are most effective against downy mildew are not necessarily effective against most other cucurbit diseases.  (A very different fungus causes powdery mildew of cucurbits.) Products that can be used to manage downy mildew are listed in the Midwest Vegetable Production Guide. The most effective products are likely to include Ranman®, Gavel® and Zampro®. Growers who have been applying Presidio® for Phytophthora blight may find that this product is also effective against downy mildew. I have found Previcur Flex® to be effective, however, more recent data indicate this product to be less effect than the above products.  Products that contain phosphorus acid (such as Agri-phos, Phostrol, Prophyt, Rappart) may in situations where the disease is not present, but may threaten in the future.  Contact fungicides such as those with the active ingredients chlorothalonil or mancozeb may not be as effective as the products listed above, but do offer some protection.   Organic growers should use a formulation of copper that is certified for organic use. 

Additional information can be found through these links.

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​Figure 1:  Downy mildew of watermelon causes a dark brown necrotic lesion often surrounded by a chloritic halo.

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Figure 2:  The spores from the downy mildew fungus-like organism may cause a 'fuzzy' appearance on the bottom of the leaf such as in the cucumber leaf shown above.  

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. 



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.  

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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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