New Cucurbit Disease Discovered in Missouri

A new disease of cucurbits was recently discovered in a central Missouri commercial field of yellow and zucchini squash. This disease, called cucurbit yellow vine disease, is caused by a phloem inhabiting bacterium, Serratia marcescens and can be transmitted by the squash bug, Anasa tristis. The symptoms of this disease were first detected on squash and pumpkin in Oklahoma in 1988 and subsequently in Texas on watermelon and cantaloupe in 1991 although the causal agent was not identified until 1997. This disease in these states has limited commercial watermelon and squash production.

Vascular discoloration due to fusarium wilt

Early season infection of cucurbit yellow vine disease causes a bright yellowing, rapid wilting, and death of young squash, pumpkin, watermelon and cantaloupe. Symptoms usually show up just as the plant begins to set fruit. In a field in which all of these cucurbits are grown, the infection is likely to show up in the squash and pumpkin before the watermelon and cantaloupe due to squash bug feeding preferences. Infections occurring on older plants usually display a bright yellowing of the leaves and do not often wilt. Because these symptoms are similar to several other problems of cucurbits, field diagnosis is often difficult.

In the mid-Missouri field where this disease was discovered this year, it is suspected the disease was present for the last two years, but was misdiagnosed as a nutrient deficiency one year and fusarium wilt in the next. Other problems that this may be confused with include bacterial wilt and squash vine borer. Unlike fusarium wilt, which causes a vascular discoloration of the lower stem (best observed in the stem about 2" above and below the soil line), no discoloration is observed with cucurbit yellow vine disease. The lower stem and roots

Early season infections of yellow vine disease can cause rapid wilting

are apparently healthy in the early stages of the infections. As the infection progresses, there may be some secondary rotting that occurs, so it is important to evaluate several plants in varying stages of decline. Bacterial wilt will also cause a rapid decline of cucurbits, but the decline compared to that of cucurbit yellow vine disease is rather gradual. Plants infected with bacterial wilt will often wilt during the heat of the day and recover at night and early morning before a total collapse is observed. The same is true for plants infected with fusarium wilt. Additionally, because cucumber beetles transmit this disease, there should have been a history of beetle infestation at some time in the season. In contrast, plants infected with cucurbit yellow vine disease will collapse quickly--seemingly overnight. To date, cucumber beetles have not been implicated as vectors of cucurbit yellow vine disease. Additionally, cucumbers are most susceptible to bacterial wilt and are not susceptible to cucurbit yellow vine disease. Cantaloupe is susceptible to both of these bacterial diseases. While bacterial wilt has been reported to infect squash and pumpkin, infections are not that common. Watermelon is resistant to bacterial wilt. Therefore, if you are trying to decide why the squash are yellowing and wilting, and the cantaloupe appear very healthy, it would be safe to assume the problem is not bacterial

Bright yellowing of summer squash infected with cucurbit yellow vine disease

wilt. Finally, cucurbit yellow vine disease could be mistaken for an infestation of squash vine borer at a distance; however, close inspection of the plants will often detect signs of the borer. Infestations usually result in the yellowing and eventual death of the lower leaves. Frass is usually present on the stem where the yellowed leaf is attached. The larvae will be inside the stem around these areas. Infestations are most common in squash and pumpkin.

Once a plant is infected with cucurbit yellow vine disease, nothing can be done. Since the bacterium over- winters in the squash bug, early season control is critical. In fields where cucurbit yellow vine disease has been a problem, it may be necessary to apply insecticides as soon as the plants emerge and to continue application through the growing season. Insecticides with the active ingredient carbaryl, pyrethrin, or rotenone are labeled for home use.

Spearheaded by a team of plant pathologists at Oklahoma State University and the USDA-Agriculture Research Service in Lane, Oklahoma, research regarding this disease is continuing. Over the last two years, this disease has been detected in Massachusetts, Tennessee, Kansas, and Colorado. We suspect that if we look, we will find many more infections in Missouri. If you suspect you may have this disease, please send in several declining plants, including the root system. If it is not possible to send in the entire plant, send in the lower stem and roots. Take a sample that includes about 6" above the soil line. Pictures of the plant in the garden or the field are always helpful.

Laura Kabrick, Extension Plant Diagnostic Clinic, UMC (573) 882-3019