May 20, 2016
Diseases challenge Louisiana tomato season

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

Singh, known as the LSU AgCenter plant doctor because he’s director of the Plant Diagnostic Center, said of the seven 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 said. 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 soil-borne diseases prevalent are Southern bacterial wilt and Fusarium wilt, Singh said. Of these two diseases, Southern bacterial wilt is devastating to tomatoes because it attacks the young plants during their vegetative growth. It spreads rapidly and may kill a healthy plant in a matter of days.

Souther bacterial whilt. Photo: Craig Roussel
Souther bacterial whilt. Photo: Craig Roussel

Southern bacterial wilt is soil-borne, and no effective chemical control is available. It is primarily managed by adopting good cultural practices. Southern bacterial wilt is recognized by green plants wilting rapidly and discoloration of vascular tissue. Wet weather and high temperatures favor disease development, Singh said.

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

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 said. 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 cultural practices.

Buckeye rot of fruit is also showing up in some parts of the state, Singh said. It is a disease caused by a soil-borne fungal-like microorganism. During rain or sprinkler irrigation, water splashes the pathogen to the low fruit, and the infection starts. Later on, the green fruit starts to rot, and lesions that look like buckeye appear on the fruit.

Phenoxy herbicide injury caused by root pick up of the herbicide. Photo: Raj Singh
Phenoxy herbicide injury caused by root pick up of the herbicide. Photo: Raj Singh

Herbicide drift injury has been a major issue this year, too, Singh said. Nontarget injury to the plants may cause leaf distortion, twisting, curling, spotting, vein bleaching and mottling. Some of the most common herbicides that cause plant injury are Roundup (glyphosate), paraquat and 2,4-D, MCPE, or Weedone (phenoxy acid).

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.

For more information on tomato diseases and their management, call Raj Singh at 225-578-4562 [email protected]

Linda Benedict, Louisiana State University

Source: Louisiana State University AgCenter

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