Last year a “new” potato disease was found in several states in the Northeast and a few in the Midwest. The bacterial disease looks at first like blackleg, caused by a different bacterium, but is much more aggressive and can cause severe losses. The disease is introduced on seed potatoes, and results in poor seedling emergence and rapid wilting in hot weather. The black stems tend not to be slimy and often do not have the rancid odor we normally associate with soft rot and blackleg caused by Pectobacterium. So far this year, Dickeya has been found in NJ, NY, DE, PA, MD, NC and VA on potatoes; in all cases seed potatoes came from Maine or New Brunswick, Canada. According to Rutgers University pathologist Andy Wyenandt, although the disease can cause 100% loss, USDA Animal and Plant Health Inspection Service (APHIS) has declared the pathogen Dickeya dianthicola non-reportable/non-actionable because it has been found in the US previously. Like most diseases caused by bacteria, management is mainly preventative, beginning with clean seed potatoes free of the pathogen. Maine certified seed potato growers are working to limit tolerances for blackleg in seed production fields. OH is one of many states that utilize Maine seed potatoes.
Blackleg symptoms caused by Dickeya dianthicola (photo by S. Johnson, Univ. Maine)
The OSU Vegetable Pathology Lab will test potato blackleg samples for Dickeya free of charge for Ohio growers. Samples can be dropped off at our lab on the OSU-OARDC Wooster campus, at OARDC Experiment Stations in Celeryville or Fremont, or mailed to us. Since samples will rot very quickly, we highly recommend they be shipped via overnight mail. Please follow instructions and use our sample submission form.
According to a 2015 Report on Horticulture by the National Agricultural Statistics Service (NASS), production of vegetables in “protected culture” nationwide increased 71% between 2009 and 2014. Protected culture is any production system in which plants are not exposed, or exposed to a reduced degree, to the elements compared to open fields. These can include greenhouses, high tunnels, hoop houses, low tunnels, etc. The protected culture of high tunnel and greenhouse tomato systems may result in lower incidence of diseases spread by rainfall such as Septoria leaf spot and bacterial spot and speck. However, some diseases that are uncommon in open fields occur often in high tunnels and greenhouses. White mold (timber rot) is among the most important of these diseases found in protected culture. The pathogen produces overwintering structures called sclerotia that can survive in soil for many years (Fig. 1). These structures, which are black in color and approximately the size of a rice grain (although they can be larger or smaller), can be found inside or outside diseased tomato stems or other tissues.
Fig. 1. White mold/timber rot lesion at base of tomato plant in a high tunnel. Abundant sclerotia can be found inside the stem (not shown), as well as outside.
Fig. 2. Ascocarps (apothecia), tiny, trumpet-like structures, arising from a sclerotium partially covered by soil.
Under the right conditions (cool temperatures, high relative humidity), the sclerotia produce tiny spore-bearing structures called ascocarps that “shoot” ascospores into the air (Fig. 2). These spores can infect flower petals and young succulent tissues, which then can serve as a source of mycelial inoculum for stems and other tougher tissues. The pathogen (Sclerotinia sclerotiorum) has a wide host range and can cause disease on almost any dicot. Timber rot can be managed primarily by employing appropriate cultural tactics. More information can be found on Vegetable Disease Facts.
- Prevent Excess Moisture– If soils remain warm and continuous wet periods are avoided, sclerotia (overwintering structures) of the pathogen will be unable to germinate. Keep plant density low and prune to increase air movement. Reduce foliage density as much as possible in the lower canopy to keep the zone between the soil and foliage as wide and dry as possible.
- Chemical Treatments– There are no fungicides labeled for use against white mold/timber rot in greenhouse or high tunnel tomatoes.
- Biological Control – Actinovate AG at 3-12 oz. per acre is labeled for greenhouse/high tunnel tomatoes to suppress white mold. It should be used with a spreader/sticker. However, we have no data on efficacy of this product against white mold. Contans is also biological control product, but it has shown good results in a number of tests. Contans should be applied to soil before or at transplanting, and again after the tomato crop to reduce the viability and number of sclerotia.
- Sanitation– Clean tools and machinery should be used in the high tunnels and greenhouses to prevent spread of sclerotia and spores. Remove fallen flower petals or dying plant material so that the pathogen cannot survive in the plant debris. Carefully dig up diseased plants and remove soil in a 4-6 inch radius around the base of the stem to remove any sclerotia that may have fallen from the plant to the soil.