121 articles tagged "Plant Disease Management".

Root knot nematode

On a sandy hillside in a watermelon field, we noted vines that, from a distance, appeared undersized compared to the vines in the flats. Upon closer inspection, some of the vines had either wilted or a portion of the plant had wilted. The wilted vines had discolored vascular tissue. These vines were affected by Fusarium wilt of watermelon. The roots of many of the plants had galls caused by root knot nematodes. That is, many of the vines were infected with both Fusarium wilt and root knot nematode. This article discusses root knot nematode on watermelon. Root-knot nematodes are small, colorless roundworms that dwell in the soil. They penetrate into plant root in the juvenile stage. Once they find a favorable location in plant tissues, they stop moving. Infested root cells start swelling and form galls that are the characteristic symptom of root-knot nematode infestation (Figure 1). Infested roots fail[Read More…]


Contact herbicide

What is causing the spots on the watermelon leaf? A) anthracnose B) early blight C) a contact herbicide   The answer is that the leaf above has been affected by a contact herbicide. The herbicide caused lesions upon contacting the leaf. However, there was no growth of the lesion and no yellowing of the leaf since the herbicide caused no effect after the initial contact. While the distribution of the lesions in the photo above appears like a disease, the lack of ridges and yellowing in the leaf above is why it is not a disease. In the photo below, which is anthracnose of watermelon, the lesions have many ridges caused by growth of the lesion. Plus, there is a bit of yellowing of the lesions.


Downy mildew of cucumber

Downy mildew has been observed on cucumber in Berrien County in extreme southwestern Michigan and in Monroe County in extreme southeast Michigan. The downy mildew spores that cause disease on cucumber will cause disease on cantaloupe and may cause disease on other cucurbits such as pumpkin and watermelon. The forecast is for the spread of the disease to move in a east to southeasterly direction. Therefore, cucurbit growers in northern Indiana should scout and manage for downy mildew. The organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana. It has to be blown in every year. It is common for downy mildew to start the season in the Gulf States and migrate north with the cucurbit crops. Downy mildew apparently overwinters in northern Michigan/southern Ontario in greenhouses where cucumbers are grown year-round. Therefore, downy mildew is often found in Michigan before it is found in Indiana. For infection to occur, free moisture[Read More…]


Figure 1. Cercospora leaf spot of beet.

This disease was observed in a home garden recently on table beet (Figure 1). I was surprised considering how dry it has been.  Cercospora leaf spot also affects swiss chard. Symptoms include circular leaf spots that may have a reddish margin. The center of the lesions may start off a light brown and turn to gray after the fungus (Cercospora beticola) begins to sporulate. Under conditions conducive to disease, the lesions can coalesce and result in loss of foliage. Yield and quality of the crop can be reduced.  Cercospora leaf spot is favored by rainy weather or overhead irrigation and temperatures from 77 to 95°F.  The spores are readily dispersed in rainy, windy weather. One reference I found said that Cercccospora leaf spot can start with 90% relative humidity—that is, leaf wetness may not be necessary.  That might be why this disease is present in this dry spell.   Cercospora may in survive crop residue over winter. The fungus may also be seed borne.  Resistant cultivars are available.[Read More…]


Question: Why are the water droplets arranged so evenly around the edge of this cucurbit leaf? Answer: The water droplets came out of pores that are at the edge of the leaf where a vein ends. The pores are called hydathodes. The droplets form through the process of guttation. Guttation is when the water pressure in the plant is high enough to force water out of the hydathodes. This occurs when soil moisture and humidity are high, typically at night during rainy humid weather. The hydathodes may also serve as a means for bacterial pathogens to enter leaves. The water droplets can be drawn back into the plant as the sun comes out, relative humidity drops, and the leaf begins to lose water through transpiration. When bacterial diseases are present, these water droplets can be a major means of disease spread if people,  animals,  equipment, wind, or rain move water droplets[Read More…]


Tomato brown rugose fruit virus (ToBRFV) is making headlines and eliciting USDA action. Growers need to learn more about ToBRFV biology, symptoms and control. Tomato brown rugose fruit virus (ToBRFV) is a newly identified virus affecting tomato, pepper and possibly their relatives. ToBRFV first appeared in Israel in 2014. Since then, it has shown up in several other countries, including eradicated greenhouse outbreaks in 2018 and 2019 in Arizona and California. These back-to-back U.S. outbreaks indicate ToBRFV will probably be something that without good diligence has a high probability of happening again. An added concern for the U.S. industry is ToBRFV is present in countries exporting tomato and pepper fruit to the U.S.; these include Mexico (where it was widespread in 2018) and the Netherlands. The virus has not been found in Canada, but some fruit imported into the U.S. goes through Canada. The U.S. Department of Agriculture (USDA) is[Read More…]


In the last issue of the Vegetable Crops Hotline, I wrote an article about common diseases of cantaloupe and watermelon transplants. Based on the samples I have received over the last few days, I would like to write about a disease that is not usually a problem: angular leaf spot. Angular leaf spot affects all cucurbits. In this article, I would like to concentrate on angular leaf spot on cantaloupe and watermelon. Symptoms of angular leaf spot on cantaloupe often consist of brown, necrotic lesions on the margin of true leaves and seed leaves (Figure 1). On watermelon leaves the lesions may appear darker (Figure 2). Under cool, wet conditions, the disease can be quite severe, resulting in hotspots where seedlings are rendered useless for field transplanting. Angular leaf spot may be seed borne. I suspect that the pathogen may also survive on transplant trays and on greenhouse surfaces. Therefore,[Read More…]


Figure 1:

Botrytis gray mold can cause disease on many different host plants, enabling the fungus to easily survive and disperse between crops. Host crops include flowers such as geraniums, vegetables such as green beans and fruit such as strawberries. The disease is favored by relatively cool temperatures and high humidity. We recently observed botrytis gray mold on tomatoes grown in a greenhouse and strawberry grown on a plasticulture system in the open field. 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 cause yield loss. Gray mold, or as it is sometimes called, Botrytis gray mold, may cause a light gray or brown necrotic lesion on leaves (Figure 1). The lesions on leaves are sometimes wedge shaped on the margin of the leaf. Stem lesions are a similar color and may encircle[Read More…]


In creating the new format of the Midwest Vegetable Production Guide for Commercial Growers 2020 (ID-56), I inadvertently left out the portion of the fruiting vegetable section that deals with Phytophthora blight of pepper. The new format allows me to update items easily; I have now added information on this important disease. Go to mwveguide.org to find the update. However, the hard copy of the Guide still lacks this information. Anyone who wants a hardcopy of the Phytophthora blight on pepper information should contact me. I apologize for any inconvenience or confusion. Please let me know of any comments or questions that you might have.


Figure 1. A common symptom of gummy stem blight of watermelon is a watersoaked area where the seed leaves attach to the stem.

1. What caused the water-soaked stem of this watermelon transplant? A-damping-off B-gummy stem blight C-Lightening strike Correct Answer: B 2. Is this problem likely to spread to other transplants? Yes 3. Will this problem likely spread in the field? Yes More information about gummy stem blight can be found in the article Cantaloupe and Watermelon Transplant Diseases in this issue.


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