High Target Spot Pressure in NC Tobacco
Persistent wet weather conditions continue to cause issues for tobacco diseases across the state. In particular, target spot, caused by Thanatephorus cucumeris (syn. Rhizoctonia solani) is severe this year. Target spot is characterized by circular light brown lesions with dark brown concentric rings within the lesion. Lesions start out as small white-light brown sunken spots, which may be easily confused with other leaf spots. As the disease progresses, lesions may have the centers fall out causing a shot-hole appearance. Warm, moist environmental conditions favor disease development, which have been prevalent across the state for most of the 2020 growing season. For more information on target spot in tobacco, see the Tobacco Disease Information page developed by Dr. Dave Shew.
Several reports of control losses have been communicated to us in Extension. Fungicide insensitivity has not yet been confirmed in NC target spot populations. We are actively investigating these reports and monitoring the severity of disease. Losses of disease control may be caused by several factors, including spray coverage, environmental conditions, rate of fungicide applied, etc. Because it takes time for new lesions to develop, scouting 5 days to a week after a fungicide application may be beneficial to observe fungicide impacts on disease spread. If you are concerned with control failures with fungicide use, please contact your extension agent for help to identify potential causes.
Managing target spot pressures
Because the weather continues to be conducive for target spot development, disease management this year may be more difficult. In general, most of the labeled fungicide chemistries require applications to be made before spores land on healthy leaves to be most effective and they have limited, if any, curative activity. Presently, the only systemic fungicide (travels both upward through the plant and translaminar mobility) labeled for target spot control in the field is azoxystrobin (e.g., Quadris). Other products like mancozeb (e.g., Manzate Pro-Stick) or organic products (e.g., Oxidate, biologicals) are not systemic, and are only effective where they are applied. It is important to make sure that the fungicide being used is accepted by the leaf buyer, and communication with the buyer about fungicide selection is important. Where possible, rotating between fungicides with different modes of action may be beneficial to reduce disease and risk of fungicide resistance development.
Regardless of product(s) used, good coverage is important for efficacy. This may mean higher water volumes, angled or drop nozzles, and appropriate driving speed. Accuracy of fungicide rates such that the minimum effective concentration of fungicide is applied per acre is also important to limit potential for sub-lethal exposure of pathogens.
Cultural practices to reduce pathogen survival and spread are always important to lessen the severity of disease. Improving airflow through the canopy, making appropriate fertilization applications to limit excessive growth, sucker control, and destruction of crop debris at the end of the growing season are all beneficial practices.
Next steps for leaf spot control
While we continue to evaluate fungal population sensitivities to fungicides, it is important to continue using disease management approaches that combines cultural and chemical strategies should help to lessen disease severity. We are also actively investigating additional fungicides that may be incorporated into standard management practices to help reduce losses from leaf spots. Alerts will be shared via the extension portals and other forms of media as we learn more about these pathosystems.