Florida and Georgia witnessed the United States’ first black shank appearance in 1915. Having crept into
Tennessee, Kentucky and North Carolina in the 1930s, black shank could be found in almost every flue-cured tobacco state by the year 2000, causing losses ranging from 1 to 2.5 percent per year in North Carolina alone.
Are these loss rates still commonplace? Mina Mila, North Carolina State University (NCSU) tobacco
specialist and associate professor in the Department of Plant Pathology, says yes. “Last year we were close to 2.5 percent,” says Mila. “Reduction of these losses is possible in the future, given better control methods, if we implement them correctly.”
Black shank (Phytophthora nicotianae) is an oomycete,
a fungus-like microbe from the same lineage as the late blight disease responsible for the infamous Irish
potato famine. The microbe favors warm and wet soils and is most prevalent in areas with poor drainage that were planted with tobacco the previous year. This makes crop rotation an essential control method in minimizing the
impact of black shank.
How long should growers wait before they rotate their tobacco back in? “The longer the better,” says Mila. “Two years is better than one. Three years is better than two. It all comes down to what the grower can afford. Black shank can’t really be eliminated if it’s there, but you can reduce it to a level that is acceptable.”
In fields with a high recurrence of black shank,
altering the terrain may be an option. “There are fields with low spots that accumulate higher concentrations of water, and, in some situations, digging a drainage ditch can help.”
Of course, chemical controls are essential, and
Ridomil Gold is a staple for all growers. “We recommend application any time from a few days before transplanting all the way to layby,” says Mila, “Though this year we have a second registered fungicide, Presidio. Presidio is applied via exactly the same method. I cannot really comment on [whether] we can see better black shank control with it until we have more years’ data, but it provides at least as good control as Ridomil.”
Black Shank Resistance
Planting with resistant varieties also aids in the fight against black shank. Behind the scenes, scores of hard-at-work scientists are breeding new varieties of tobacco with better traits. NCSU is a global leader in tobacco breeding, and Ramsey Lewis, associate professor of crop science, is a distinguished leader in the NCSU breeding program and a world authority in tobacco breeding.
“Historically, we haven’t had a lack of good black shank-resistant varieties,” says Lewis. “There are a number
of varieties—K346 for example—that have high levels of black shank resistance. What we have been lacking is high-yielding black shank-resistant varieties.”
Because varieties like NC196 and K326 provide much better yield, the best black shank defenders fall by the wayside. “Last year, NC196 was planted in more than 50 percent of flue-cured acreage in North Carolina,” says Lewis. “For the past five years it has been an excellent variety, though disease-
resistance improvements can definitely be made.” And this is where breeders’ research has been
focused: creating varieties with excellent disease resistance that post yield numbers
comparable to today’s most popular varieties.
NC938 is an example of an exceptional variety produced by that research. Though it is still experimental, it will be commercially available in 2016 and beyond. “NC938 is an example where the black shank resistance is not in doubt. We can’t put a number on its yield
potential yet since it’s still in testing, but we expect yields to be very good.”
Growers can expect even better varieties in the near future. Over the past 10 years, Ramsey and his colleagues have been exploring other sources of resistance,
including wild varieties, and those efforts are nearing commercialization. In addition to producing better yields, these new varieties may reduce management costs; in certain situations, they could decrease the levels of fungicide application necessary to manage disease. So keep on the lookout for these improved
varieties as they are released over the next few years, and if you find yourself looking out across a
healthy field, don’t forget to thank a breeder.