Garden News: Experts keep eye on tomato diseases _lowres

Photo by CRAIG ROUSELL, LSU AgCenter – Southern bacterial wilt is devastating to tomatoes because it attacks young plants during vegetative growth.

LSU AgCenter plant pathologist Raj Singh says he has never seen so many different diseases of tomatoes in one season.

Known as the LSU AgCenter plant doctor, the director of the Plant Diagnostic Center says of the diseases reported, the first he heard about earlier this season was white mold, also known as timber rot.

White mold is a soil-borne fungal disease and favored by extended periods of cool, wet weather, he says. Infected plants wilt rapidly, collapse and die.

As the disease develops, infected green tissue bleaches out and becomes off-white. The disease is characterized by the presence of white cottony fungal strands with hard black irregular structures (sclerotia) at the infection sites. Two other prevalent soil-borne diseases prevalent are Southern bacterial wilt and fusarium wilt.

Southern bacterial wilt is devastating to tomatoes because it attacks young plants during vegetative growth. It spreads rapidly and may kill a healthy plant in a matter of days.

Southern bacterial wilt is soil-borne, and no effective chemical control is available. It’s primarily managed by adopting good gardening practices. Bacterial wilt can be recognized by rapidly wilting green plants with discolored vascular tissue. Wet weather and high temperatures favor disease development, Singh says.

Fusarium wilt is a late-season disease that kills the plant gradually. Infected plants wilt, exhibit one-sided yellowing of the leaflets and internal discoloration of the vascular tissue.

Another bacterial disease that has been showing up is pith necrosis. Symptoms start with yellowing of young leaves, followed by wilting of infected shoots in the upper part of the plant canopy.

Black lesions appear on petioles and stems, which may shrink, crack or collapse, and pith of symptomatic stems becomes hollow. Adventitious roots develop profusely on the affected stems.

Among foliar diseases, early blight and bacterial speck and spot are becoming prevalent in both home gardens and commercial production, Singh says. High humidity, free water on the foliage and poor air circulation favor foliar diseases.

Early blight produces large brown necrotic spots with concentric rings. Bacterial speck and spot produces small irregular brown necrotic spots, usually surrounded by a yellow halo.

Foliar diseases can be easily managed by preventive spraying of labeled fungicides or bactericides incorporated with good garden management. Because the weather is conducive for disease development, tomato growers may start to see Southern blight during the next several weeks when tomatoes are at ripening stages.

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.