News from the OSU Vegetable Disease Diagnostics Lab

It has been a busy week in the diagnostics lab – we continue to receive high tunnel and greenhouse tomatoes, but open-field vegetables and herbs are now making an appearance. These are some of the plant disease problems from around Ohio received this week.

Basil downy mildew – sporulation of pathogen Peronospora belbahrii on stem

Basil downy mildew. The sample from Ashland County had unusual sporulation on the stems (see photo). We normally find lesions on leaves with spores on the underside. This might indicate a systemic infection, possibly from seeds. See my post on May 4, 2017 for recommendations on management of this disease. Note: The basil downy mildew pathogen does not cause disease in cucurbits. We have not seen cucurbit downy mildew in Ohio or in the Great Lakes region yet this year.

Early symptoms of bacterial wilt on cucumber leaves – photo by F. Rotondo

Cucumber bacterial wilt. This disease was found in Highland County. Bacterial wilt can be a very serious problem in cucumbers, melon, squash and pumpkins, among other cucurbits. It is caused by a bacterial plant pathogen – Erwinia tracheiphila – that overwinters in striped and spotted cucumber beetles. When the beetles emerge from the soil they feed on cucurbit plants and transmit the bacteria to wounded tissue. We expect that beetle survival was higher than usual this year as a result of the mild winter of 2016/2017. The beetles can damage or kill young plants even in the absence of Erwinia, but if the bacteria enter the plant it is likely to die or at least perform poorly. The earlier plants are infected, the more likely they are to die quickly. Conventional growers can control bacterial wilt with appropriate insecticides – see the Midwest Vegetable Production Guide (pp. 122-123 in 2017 Guide) for recommendations. Options are limited for organic growers and gardeners, although row covers until flowering will protect plants when they are most vulnerable.

Brown longitudinal lesion and root rot of eggplant caused by southern blight; white mycelium of the pathogen Sclerotium rolfsii

Southern blight. We found this disease on young eggplant and pepper seedlings on OSU’s Waterman farm in Columbus. This is a disease that thrives under hot, moist conditions – it is not common in northern Ohio but we have seen it before in central and southern Ohio. An elongated brown lesion on the lower stem and root rot were accompanied by white mycelium (see photo), sometimes clearly in a fan shape, at the base of the stem. Small, round sclerotia eventually form on the surface and serve as overwintering structures. Sclerotia can survive many years in the soil, so it is important to remove diseased plants and accompanying sclerotia from the field.

Lesions of Septoria blight on tomato seedlings

Septoria blight of tomato. The disease was diagnosed tomatoes from Highland County. Typical symptoms are small necrotic spots on leaves, but not on fruit, which can help distinguish this disease from bacterial spot. Necrotic lesions caused by Septoria lycopersici contain tiny round black structures called pycnidia that can be seen with a hand lens. The pycnidia contain spores of the pathogen that are released when exposed to water. Therefore the disease is rare in greenhouses and high tunnels unless there is exposure to rainsplash or overhead irrigation. Septoria can be seedborne – we diagnosed Septoria blight in tomato transplants from a greenhouse with overhead irrigation earlier this spring in which infested seed was a likely source. Septoria blight is best managed with timely fungicide applications; a strobilurin fungicide should be included in the program.

Severe 2,4-D damage on tomato

Herbicide drift damage – high tunnel tomato. We have seen several cases of herbicide drift damage to tomatoes from high tunnels so far this season. When we receive samples that look like possible herbicide damage, we pass the sample on to OSU’s Dr. Doug Doohan. He diagnosed the sample pictured here as damage from exposure to the herbicide 2,4-D. This was a very severe case.

Corky root rot of tomato

Corky root rot of high tunnel tomato. This disease is often part of a soilborne disease complex of tomato that may also include Fusarium wilt, Verticillium wilt, black dot, and root knot nematode, among others. In this case, corky root rot was the primary problem. Like other soilborne diseases, corky root rot is very difficult to manage if resistant varieties are not available. Where the disease is a problem, grafting onto a corky root rot- resistant rootstock is very helpful in managing the disease. We are also working on anaerobic soil disinfestation (ASD) as a method of reducing populations of soilborne plant pathogens in high tunnels.

Many thanks to OSU Vegetable Pathology lab members Dr. Francesca Rotondo, Dr. Anna Testen and Claudio Vrisman for their work on these cases.

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