We are seeing many reports of yellowing and/or wilting squash, pumpkin and other cucurbits in commercial fields and gardens this month. Chances are that the cucurbits have been affected by cucurbit yellow vine decline (CYVD) or bacterial wilt. Both of these diseases are caused by bacteria transmitted to plants during the feeding of their insect vectors. Once infected, the plants cannot recover; these diseases must be managed preventatively by controlling the insect vectors, ideally early in the season. At this point, insecticides may be applied to prevent the diseases from spreading to healthy plants. It is also useful to remove and destroy symptomatic plants that serve as sources of bacterial inoculum.
Sticky bacterial ooze from a cucumber vine with bacterial wilt
Bacterial wilt of melon
Bacterial wilt affects many cucurbits including cucumber, melon, pumpkin and squash. The causal agent, Erwinia tracheiphilia, overwinters in the digestive system of spotted and striped cucumber beetles. When these beetles emerge in the spring, the pathogen is spread from beetle feces to healthy cucurbits mainly via wounds caused by insect feeding. Bacterial wilt occurs almost every summer in Ohio, but is less severe after very cold winters that reduce overwintering beetle populations. Symptoms begin as discoloration and wilting of individual leaves. As the disease progresses, the entire plant begins to wilt and collapse as the bacteria clog the xylem vessels. When the stem is cut along the base, clear to white elastic strands comprised of the bacteria and “gum” are visible when cut ends of stems are slowly pulled apart. More details can be found here. Insecticides labeled for cucurbits and effective against the beetles can be found in the Midwest Vegetable Production Guide for Commercial Growers. Covering plants with netting or floating row covers until flowering can also protect plants from early infection.
Cucurbit yellow vine decline in summer squash
Cucurbit yellow vine decline in pumpkins.
Cucurbit yellow vine decline (CYVD) is caused by the bacterial pathogen Serratia marcescens, transmitted by squash bugs. It is uncommon in some years but in others can do a lot of damage. Bright yellowing of leaves, followed by by wilting and death of plants is indicative of possible CYVD. If squash bug adults, nymphs or eggs are found on the underside of leaves, this is a good clue that the symptoms are caused by CYVD. A cross-section of the vine may show a light tan discoloration of the vascular tissues. CYVD is managed by applying insecticides (see Guide link above). For both CYVD and bacterial wilt, fields and gardens should be scouted regularly for the insect vectors beginning soon after transplanting or seedling emergence.
The OSU Plant Disease Diagnostic Clinic in Wooster can provide a definitive diagnosis of CYVD using a PCR assay. Testing for this and other diseases and pests is free of charge for Ohio commercial vegetable growers thanks to financial support from the Ohio Produce Growers and Marketers Association.
Eggs of the squash bug
Cross-section of squash vine with light browning of the vascular system caused by the bacteria