Unseasonably warm weather followed by cool, damp conditions has led to an increase of tobacco diseases in float beds. Instances of target spot, collar rot and Pythium root rot are showing up in float systems across the state.

"Growers need to watch their seedlings closely for any sign of these diseases so they can manage them if they develop," said Kenny Seebold, extension plant pathologist with the University of Kentucky College of Agriculture.

Target spot and collar rot develop and spread during damp, cool weather. These diseases likely came in around April 16 with a cold front that brought clouds and rain to the area. The following weekend provided the prime opportunity for those two diseases to thrive in greenhouses.

The first sign of target spot is green lesions on the plant leaf. These lesions can turn brown and brittle if left untreated. The lesions can lead to defoliation, slow maturity, damping off and, in some cases, plant death.

To prevent the disease, burley growers should limit the time transplant leaves and stems are wet by using ventilation and checking float bed water levels to maximize air flow in the greenhouse. Regularly clipping plants will also improve ventilation, but growers should remove any clippings so other diseases like blackleg and collar rot do not appear later in the season. Target spot tends to be worse in plants with low nitrogen levels, so maintaining adequate nitrogen levels in seedlings will help with disease prevention or at least lessen its effects. 

Growers can use fungicides with the ingredient mancozeb to prevent this disease when conditions are favorable for its development.

Recent cool temperatures, high humidity and overcast skies were ideal for the development of collar rot, and several cases have already been reported. This disease is caused by a fungal plant pathogen and can be identified by small, dark green, water-soaked lesions appearing at the bases of stems. Evidence of the disease is most visible when clusters of infected transplants collapse, leaving holes, usually about the size of grapefruits, in the plant canopy.

Plants between 5 and 7 weeks old are the most susceptible to this disease.

No fungicides are labeled to control collar rot on tobacco transplants. Therefore, growers need to use sound management practices to prevent and control the disease. Just as in target spot prevention, plants need adequate ventilation and air circulation. Frequent clippings also help. Growers should bury or discard clippings and diseased plants at least 100 yards away from the greenhouse. They also should not plant home gardens near the greenhouse and keep float beds free of weeds because more than 300 species of plants and weeds are hosts to the collar rot-causing pathogen.

While it was wet this past weekend that has not been the case for the majority of the month. Above-normal temperatures during the end of March and first two weeks of April led to the development of Pythium root rot. The disease develops and spreads when float bed water temperatures reach and exceed 72 degrees.

"The first symptoms of Pythium root rot tend to be yellowing and stunting of transplants in a well-defined area or areas of a float bay; however damping-off can occur in severe cases," Seebold said. "During the outbreak, seedlings wilt and root systems decay to some degree."

Sanitation is key to preventing Pythium root rot. The disease is most commonly introduced into the float system through contaminated surface or pond water, infested soil or recycled trays.

To prevent Pythium root rot in float systems, producers can use Terramaster 4EC two to three weeks after seeding. Additional treatments can be made at three and five weeks after the first fungicide application, but it should be applied no later than eight weeks after seeding. It can also be used to treat infected plants but may not completely kill the disease. Growers should refer to the product's label for specific instructions and safety information before making an application.