Mid-Season Tobacco Disease Updates

— Written By Lindsey Thiessen

Recent weather conditions have favored several diseases in tobacco. Most pathogens that we manage in tobacco are most severe under warm, wet conditions. In 2020, I am receiving continued reports of vascular diseases and leaf spots affecting tobacco.

Vascular Diseases

Several Plant Disease and Insect Clinic confirmed reports with Granville wilt (Ralstonia solanacearum) and black shank (Phytophthora nicotianae). Symptoms of disease can look similar. Granville wilt is characterized initially by a one-sided wilt, and later in the disease progression causes the entire plant to wilt and die. Because it is caused by bacteria, Granville wilt stems will have bacterial oozing when cut stems are placed in water.

Black shank is caused by a water-mold (oomycete), and causes an entire plant wilt. Lower leaves may also have leaf spots from where fungus-infested soil was splash dispersed to leaves. When stems are cut in a cross-section, disking and blackening of the pith may be observed.

Once symptoms are observed, there are no remedial controls for black shank or Granville wilt. More information on disease management can be found on the fact sheets for Granville wilt and black shank.

Foliar Diseases

Target spot (Thanaetophorus cucumeris syn. Rhizoctonia solani) has been reported across the state, especially in areas that have head heavy precipitation over the last several months. There are several things that may help improve disease severity in fields affected by target spot. Improving canopy air flow and maintaining adequate drainage in the field will limit humidity in the canopy. Fungicide applications may be necessary to prevent spread upwards through the canopy.

Though frogeye leaf spot, brown spot, and angular leaf spot have not yet been confirmed this year, they are likely in fields at low levels. These are also favored by warm, wet conditions, and care should be taken to scout fields for their presence as well. Fungicide resistance has been documented in frogeye leaf spot (Cercospora nicotianae), and azoxystrobin alone may not be adequate in reducing populations. Employing cultural practices like those described above for target spot may help to limit frogeye leaf spot disease incidence.

Season Outlook

Although projections show continued wet conditions, periods of dry weather may certainly be on the horizon. Symptoms of diseases like black shank, Granville wilt, and those caused by nematodes are most noticeable during periods of drought. Scouting is incredibly important to identify problem fields. Sanitation of equipment between fields will help to limit spread of these and other pathogens between production sites.

I unfortunately do not have a crystal ball to predict what will come next given the variability in weather patterns throughout the state; however, we can plan to be vigilant in scouting and management decisions. For more assistance in diagnostic support, contact your cooperative extension agent. Submission of physical samples or image samples to the Plant Disease and Insect Clinic may also be beneficial in identifying problems in fields.