Aug 7, 2015
Why are my cucurbits wilting?

This season has been rough on cucurbit crops across the state and many growers are seeing cucurbits wilting in their fields. There are a number of diseases like bacterial wilt, cucurbit yellow vine decline and Fusarium wilt as well as abiotic stresses such as water logged soils that can cause cucurbits to wilt.

Below is a description of a few diseases that can lead to wilting. In all these cases, the wilting is a result of the pathogen colonizing the vascular system and preventing the plant from moving water and nutrients from the roots to the shoots. Although there not much that can be done once you see symptoms in the field, it is important to determine the cause to inform your management decisions for next growing season whether it be changes in your crop rotation, selection of resistant varieties, etc. It is often difficult to distinguish these diseases in the field so consider submitting a whole plant sample to the Penn State Plant Disease Clinic for diagnosis.
Fusarium Wilt
Fusarium wilt most commonly affects watermelon, muskmelon and cucumber. The fungal pathogen Fusarium oxysporum is very host specific so watermelon, muskmelon and cucumber are all susceptible to their own unique strain of the pathogen. On more mature plants, the leaves will initially become a dull green to gray-green and the older leaves more yellow. The shoot tips will wilt then become brown and dried up with individual vines on the plant collapsing and eventually the entire plant collapsing. Symptoms are most severe when the plants are stressed or during fruiting. One diagnostic characteristic is the light to dark brown discoloration of the xylem tissue in the crown and lower portion of the stem. This pathogen can persist in the soil for many years as chlamydospores. Since Fusarium is a true soilborne pathogen, the symptoms will tend to develop in hot spots in the field which will slowly expand over time with each susceptible crop and as tillage and field preparation practices move soil within the field.
Verticillium Wilt
Verticillium wilt, which is most problematic on cucumber and pumpkin, has a very wide host range including over 400 different hosts. Symptoms typically develop during fruit set with the lower leaves initially becoming off green and wilting of the lower leaves. These leaves eventually become necrotic and dry up. With Verticillium wilt, the symptoms are often observed on only one side of the plant. As with Fusarium wilt, the xylem tissue will become tan to brown in color. The pathogen can survive in the soil many years, sometimes as long as 8 to 10 years, as microsclerotia that are the size of poppy seeds.
Bacterial Wilt
Bacteria wilt is caused by the bacterial pathogen Erwinia tracheiphila which is vectored by the cucumber beetle. It is most damaging on cucumber and muskmelon, less so on pumpkin and squash and not a considered problem on watermelon. Symptoms initially include individual leaves that become off-green in color and wilt before turning necrotic brown and drying up. Eventually entire vines can collapse. Cucumber beetle feeding on individual leaves can result in water soaked pale green and then necrotic lesions on the leaves. Sticky strands of bacterial ooze can sometimes be observed when the cut ends of a cucurbit stem are pulled apart, however absence of the sticky ooze does not mean absence of disease. The sticky ooze can variable depending on the environment and cucurbit host. The key to managing bacterial wilt is managing the cucumber beetle early in the season. Individual symptomatic plants are often are scattered across the field rather than occurring in hotspots like some of the other wilt diseases due to the distribution of the cucumber beetle and the fact that not all cucumber beetles contain the bacterial pathogen.
Cucurbit Yellow Vine Decline
This disease is caused by the bacteria, Serratia marcescens, which is vectored by the squash bug (Anasa tristis). The symptoms are similar to bacterial wilt which is transmitted by the cucumber beetle but disease progression is much more rapid. The plants can wilt and turn yellow almost overnight usually 10 to 14 days before the fruit is mature (see picture). Cross-sectioning of the crown can reveal discoloration of the phloem tissue which has become colonized by the bacteria. The key to managing CYVD is early detection and management of the squash bug in the nymphal stages. Similar to bacterial wilt, once the bacteria are inside the plant there is little that can be done to prevent the plant from dying. The squash bugs themselves can also cause direct damage by using the piercing-sucking mouthparts to suck the sap out of the leaves causing them to wilt and collapse. Intense feeding can cause the entire leaf to collapse. Although the squash bug is considered a pest on all cucurbits, it prefers squash and pumpkins and CYVD is primarily a problem on these cucurbit crops as well.

— By Beth K. Gugino, Associate Professor, Vegetable Pathology, Penn State University

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