Blue mold, one of the most economically debilitating diseases of tobacco, comes and goes across the upper Southeast. In 2010 it came quickly, but was met with a quick response from growers, which seems to have contained spread of the disease.
The outbreak was first detected in late May in a burley tobacco field near Lucama, N.C. Young tobacco plants in the field had severe blue mold lesions and most lesions had active sporulation, indicating potential spread of the disease.
The NC5 variety burley tobacco was destroyed and adjacent fields were sprayed with a preventative fungicide (Quadris). Flue-cured tobacco in an adjacent field showed symptoms of blue mold and was likewise treated with a fungicide.
In early June, blue mold was reported in Nash and Johnson counties farther to the east, followed by additional reports of the disease near Washington in Beaufort County. The latest report of the disease came in mid-June in Beaufort County.
In the latest outbreak, about 2 percent of a 25 acre field showed symptoms of a recent infection. Lesions were new and sporulation was reported as being light.
Though the disease spread rapidly to eight counties in northeast North Carolina, North Carolina State University tobacco specialists say the outbreak appears to be subsiding as growers responded quickly to treat tobacco with fungicides in the area and tobacco advanced to growth stages less likely to be affected by the disease.
Blue mold develops in cool, wet weather, usually early in the growing period for tobacco. It can affect transplants in the greenhouse and rapidly spreads to the field once planting begins.
The disease is characterized by circular yellow spots, most often found on the upper surface of tobacco leaves. The disease name comes from the blue-gray fungus that sporulates on the underside of these leaf lesions.
Spots later turn brown, die and may fall out, giving a ragged appearance to the leaf. Blue mold damage done to the tobacco leaf can render it worthless, but almost always reduces the premium price growers receive for their crop.
In severe situations, blue mold may also cause systemic stem infections — as were reported in the early 2010 outbreak in North Carolina. Systemic infections can cause partial or overall stunting of the plant, with narrow, mottled leaves. Tobacco plants often lodge or break, if systemic infections occur near the base of weakened stems.
Asimina Mila, an assistant professor and Extension specialist at North Carolina State University, says quick action by county Extension personnel and farmers in the 8-10 county area of North Carolina impacted by the 2010 blue mold outbreak, has minimized damage to this year’s crop and slowed the spread of the disease.
Mila notes all blue mold collections in recent years have been resistant to mefenoxam (Ridomil). The development of Ridomil-resistant strains of the causal fungus has made control of the disease much more difficult.
The North Carolina State researcher says there are some production practices growers can use to help with blue mold control. These include:
• Manage your own crop as much as possible.
• Make the environment less favorable for the pathogen to survive and infect your tobacco.
• Keep the pathogen out of tobacco and the area for as long as possible.
• Protect tobacco plants with fungicides when they are most vulnerable. Ridomil will not control blue mold anymore. It is, however, of value for controlling Pythium diseases and black shank.
• Manage the crop to harvest quickly, but not prematurely.
• Don't forget about other diseases; target spot is often mis-diagnosed as blue mold.
Growers also have a couple of fungicide options to treat tobacco infected by the blue mold-causing fungus or to prevent the disease from forming.
When blue mold warnings are issued, growers should begin weekly sprays with tank-mixes of Acrobat 50WP (or Forum) and Dithane DF Rainshield. Preventative sprays can be applied, making certain to obtain complete leaf coverage — both top and bottom of the leaf surface.
High pressure sprayers (100-250 psi) and hollow cone nozzles should be used to insure maximum leaf coverage.
Preventing economic loss from blue mold is a combination of good production practices and scouting and staying aware of potential blue mold outbreaks and timely application of fungicides.