While 2015 was a light to moderate year for black shank of tobacco, 2016 is proving to be much more challenging. Black shank can be the most yield-limiting disease of tobacco, and once a field is infested with the pathogen, it can persist in soils for several years. A combination of management approaches is necessary for adequate black shank control, including crop rotation, selection of resistant varieties, and transplant water and soil-directed fungicides.
Black Shank Basics
Black shank of tobacco is caused by the oomycete pathogen Phytophthora nicotianae. The most common way farms acquire this pathogen is through soil or water movement. This can include soil movement on tractor tires, cultivators, fertilizer spreaders, contaminated tools, and even workers’ boots. The black shank pathogen primarily infects plants through the roots, which differs from the blue mold or target spot pathogens, which primarily infect through the leaves.
Symptoms of black shank start as wilting during the heat of the day, though early on plants may appear to recover overnight (Figure 1). Later, plant leaves become yellow (chlorotic), and do not recover from wilt even under conditions of adequate moisture (Figure 2). These plants are often stunted (Figure 2) and often have blackened tissue within the stem at the root-stem interface (Figure 3). In late stages of the disease, plants die back completely and in severely affected fields, yields are significantly reduced (Figure 4).
Adequate management of black shank of tobacco requires a three-way approach.
- Rotate fields away from tobacco for 3 to 5 seasons to reduce levels of the black shank pathogen. While this pathogen can never be truly eradicated from fields, it can be significantly reduced by growing non-host crops over several seasons. nicotianae, the black shank pathogen, can infect other crops, but it is maintained and increases best in tobacco. A number of common rotational crops, including grains, are poor hosts for this pathogen.
- For fields with any history of black shank, choose a resistant variety. Very good to excellent resistance to black shank has been bred into a number of recent burley releases, including KT-204, KT-206, KT-209, and KT-210. Good to moderate resistance is available in the dark tobacco varieties KT-D14, PD 7305, DT 538, DT 558, and KT-D8. Two different races of the black shank pathogen are common in Kentucky, and unless specific farm populations have been tested, the most conservative approach is to choose a variety with resistance to both races.
- Apply fungicides in transplant water as well as soil-directed sprays. Fungicides available for application in transplant water include Ridomil Gold and Orondis Gold. At first cultivation and layby, Ridomil Gold, Presidio, and Orondis Gold may be used. Post-transplant applications are most effective when directed at soils using drop nozzles followed by cultivation to move the fungicide into the root zone. Under ideal conditions, this would be followed by a light rain to facilitate fungicide uptake into plant roots.
- Burley and Dark Tobacco Production Guide (ID-160)
- Fungicide Guide for Dark and Burley Tobacco, 2016 (PPFS-AG-T-08)
By Emily Pfeufer, Extension Plant Pathologist