By Kenny Seebold, UK Plant Pathologist
Above-normal temperatures have prevailed from the end of March through the first two weeks of April, and we've been pretty dry as well. With the increased heat, we'd naturally worry a little about Pythium becoming active on roots of tobacco, and the risk from bacterial soft rot (black leg) would be elevated as well. The last things that we'd think would be a problem would be target spot and collar rot, yet both diseases are starting to crop up around various parts of the state. The appearance of these diseases more-or-less coincides with the passing of a cold front around April 16, when we also saw a little cloudy and rainy weather. With the cooler temperatures, and rain forecasted through April 27, we should expect a fair number of cases of target spot to crop up. An article on managing target spot was published in the Kentucky Pest News (No. 1225, April 13, 2010); this week's report will focus on the identification and management of collar rot.
BACKGROUND. Collar rot shows up in float beds in the spring, when resting structures (sclerotia) located outside the float system germinate produce cup-shaped fruiting bodies called apothecia. Apothecia then produce spores (ascospores) that are dispersed on wind currents. When ascospores land on susceptible tissue, they germinate if sufficient moisture is present. Long periods of leaf wetness (greater than 16 hours) are required for this process. Germinated ascospores produce hyphae (fungal "threads") that penetrate tissue and begin the infection process.
SYMPTOMS. The first symptoms of collar rot are small, dark green, water-soaked lesions that appear at the bases of stems. In most cases, this disease becomes apparent when clusters of infected transplants collapse, leaving open holes in the plant canopy. These clusters, or "foci", are usually grapefruit-sized (4-6" in diameter). Stems of affected seedlings generally show a wet necrosis that is amber-to-brown in color, beginning at the base of the plant and extending upward. Signs of the causal agent, Sclerotinia sclerotiorum, may be present on symptomatic plants or on debris in float trays. These signs include a white, cottony mycelium (fungal mass), present if humidity is high, and irregularly shaped, black sclerotia. Sclerotia resemble seeds or rodent droppings and are the primary survival structure of S. sclerotiorum and are the primary source of inoculum for outbreaks in subsequent years.
Plants that are 5-7 weeks old are most susceptible to collar rot. We often see the first cases shortly after plants are first clipped following a period of disease-favorable weather. Cool temperatures (60 to 75 ºF), high humidity, and overcast conditions, like those that have been common in Kentucky for the past week, are ideal for development of this disease. It's also important to note that S. sclerotiorum is an efficient colonizer of dead plant matter and weakened or injured tissue, and these are usually the first to be attacked. The fungus will then move from these areas to nearby healthy plants as long as cool temperatures and high humidity prevail. This is one of the ways that secondary spread of the collar rot pathogen takes place, since S. sclerotiorum does not produce airborne spores on infected tissue. The other way in which secondary spread can occur is through dispersal of infected tissue – a possible event when infected plants are clipped.
MANAGEMENT. There are no fungicides labeled for control of Sclerotinia collar rot on tobacco transplants, making this a difficult disease to manage. Sound management practices are the only options that a grower can use to fight collar rot. Adequate ventilation and air circulation are a primary concerns, since these limit the duration of leaf and stem wetness. Growers should manage temperatures to promote healthy plants and minimize injury. The latter is important because injured tissues are more susceptible to S. sclerotiorum. Fertility should be kept at around 100 ppm (N); excessive levels of N can lead to a lush, dense canopy that will take longer to dry and will be more susceptible to attack by the collar rot pathogen. Plant debris should not be allowed to build up in transplant trays or remain in contact with seedlings. Clip seedlings at a low blade speed with a well-sharpened, high-vacuum mower to ensure complete removal of leaf pieces in the least injurious way possible. Frequent clippings will reduce the amount of tissue that must be removed by the mower and will cause less plant injury and lead to less leaf material left on the transplants. Clippings and diseased plants should be discarded a minimum of 100 yards from the transplant facility, or buried. Home gardens should not be planted near transplant facilities, and keep a weed-free zone around float beds. Over 300 species of plants, including many weeds, are hosts to S. sclerotiorum, making many weeds potential hosts for this pathogen.
