Sclerotia of Slerotinia White Mold (Timber Rot) on a tomato stem.
The pathogen that causes timber rot of tomato is the same one that causes white mold on snap bean and soybean and has a host range of over 170 crops including many vegetables. Pathogen spread and disease development is greatly influenced by the prevailing weather conditions and production practices that contribute to poor air circulation and moisture retention as well as successive years of cropping susceptible hosts will increase pathogen density in the soil. Timber rot is a cool, moist-weather disease that is especially severe when the temperatures are between 59 and 70˚F under high humidity, rainfall and/or heavy dew. In 2009, the cool and wet conditions that prevailed across Pennsylvania were very favorable for white mold across many susceptible hosts and may have increased pathogen populations in those fields that then serve as a source of the pathogen for this season.
How do I know if my tomato plants have this disease?
White mold/timber rot typically initiates either in a leaf axil, where a flower blossom falls and becomes lodged providing a nutrient source or on damaged stems or petioles. The pathogen does not infect healthy tissue until after it has colonized dead or senescent plant parts such as flowers or leaves. The lesions initially have a water soaked appearance and will gradually enlarge to cover the stem. Older lesions will eventually become bleached and light gray to brown in color. When environmental conditions favor the pathogen, dense white mycelium will develop on the surface of the lesions and large black sclerotia will develop on the outer surface of the lesions or within the stem.
Is this disease seed-borne or does it survive in my soil? (Where did it come from?)
Timber rot is caused by the fungal pathogen Sclerotinia sclerotiorum. It produces sclerotia which are hardened masses of mycelium that have thick melanized rinds and are highly resistant to degradation. This overwintering structure allows the pathogen to survive in the soil in the absence of a host for as short as a few months to as long as 10 years depending on the soil conditions (temperature, moisture and depth). In response to environmental conditions, the sclerotia within the top 1 to 2 inches of the soil surface will germinate and produce a mushroom type structures called apothecia that will release ascospores that are carried by the wind. The ascospores are very sticky and if protected they can survive on the leaf surface for up to 2 weeks. Sclerotia buried deeper in the soil profile will not germinate until tillage or another form of soil disturbance brings them to the soil surface. Secondary spread during the season only occurs when there is direct contact between the symptomatic tissue and healthy tissue. Keep in mind that the pathogen must first become established on senescing plant tissue before it can infect and colonize the tomato plant. It requires a food source in order to produce the enzymes that are used to break down and degrade the plant tissue.
There are four primary ways that fields can become infested with sclerotia. The most common is when susceptible crops and weeds become infected by ascospores coming from adjacent infested fields. The pathogen is able to then complete its lifecycle and produce sclerotia that infest the field. Wind-blown infested soil or crop debris and/or contaminated equipment can also introduce sclerotia into the field. Surface irrigation water or rain can move sclerotia between fields. Sclerotia can also be introduced into a clean field via infested seed. This is most common when infested fields are harvested for seed and due to the size and density of the sclerotia they are not separated from the seed.
I always purchase my tomato plants from a transplant producer. Do I need to start growing my own in order to avoid this disease?
The timber rot pathogen is not typically thought to be spread via transplants the way other diseases such as bacterial canker are. However, it is still important to have a good relationship with your transplant supplier and make sure that they have a good disease management program in place that emphasizes proper sanitation and environmental management.
Are there other plants that get this disease?
Sclerotinia sclerotiorum has a world-wide distribution and a very large host range that includes over 408 plant species including many vegetables and common weeds such as snap bean, cabbage, potato, tomato, as well as alfalfa, soybean, sunflower, canola. Susceptible weeds include lambsquarters, pigweed, Canada thistle, and wild mustard and can plant a critical role in the disease cycle.
In the Mid-Atlantic Commercial Vegetable Production Recommendations guide Contans WG is listed for managing timber rot. When and how do I use this materials and is it effective at managing this disease?
Contans WG is a biocontrol product that contains the active ingredient, Coniothyrium minitans, a fungus that parasitizes the sclerotia over a period of several months. The product is a living spore suspension that is sprayed and watered into the soil and onto crop residue. To allow time for the biocontrol fungus to parasitize the sclerotia, it must be applied well in advance of planting the crop and/or in the fall following harvest. Coverage and incorporation is critical because the biocontrol must come in contact with the sclerotia. It is most effective when used as part of a larger integrated pest management program. Following application avoid any type of deep tillage that could potentially redistribute sclerotia from deeper in the soil profile closer to the soil surface. See the Contans WG label for additional application instructions. Contans WG is OMRI approved.
Fontelis which contains penthiopyrad (FRAC code 7) and Botran which contains dicloran (FRAC code 14) are both labeled for Sclerotinia in greenhouses. Endura contains the active ingredient boscalid (also FRAC code 7) is specifically labeled for Botrytis gray mold but it will also help manage timber rot. In order to provide any level of efficacy, it must be applied prior to the observation of symptoms at flowering. and is labeled for greenhouse use.
I need to plant tomatoes back into the same high tunnel where I lost plants to timber rot this year, what would be a good strategy to use to manage this problem in the future?
No single management tactic will effectively manage timber rot however applying an integrated approach will help to minimize potential losses and further build-up of the pathogen. Rogue out symptomatic plant parts as they are found during the season to minimize the potential development of sclerotia. Consider removing the soil immediately surrounding the stem (approx. 6 to 8 inch radius and 4 to 6 in. deep) that may contain sclerotia and bury in another location not being cropped to a susceptible host. In the high tunnel, maximize air flow in and around the plants by increasing in-row spacing and trellising the plants. Optimize fertility to avoid dense lush vegetative growth. Planting into plastic mulch and mulching the row middles will help to create a barrier between the soil and the crop however this will not prevent wind-blown ascospores from entering the high tunnel.
The infective spores can originate both from within the high tunnel or can be blown in from outside the tunnel so sanitation and pathogen management is also important to manage the area within 330ft of the high tunnel or greenhouse for the pathogen as well. Many weeds including Canada thistle, lambsquarters, mustard, nightshade, pigweed, ragweed, velvet leaf and vetch are all hosts so weed management is critical. Timber rot is favored by cool moist conditions. Be on the look-out for timber rot when the daytime highs are consistently in the 70's or lower. Blooming periods are especially critical for pathogen infection because it first colonizes senescent tissue before infecting healthy stems and petioles. Timber rot is a mono-cyclic disease which means that critical time for management is during the period of ascospore release from the apothecia and during flowering when the crop is most susceptible.
Contans WG is a biocontrol product that has been demonstrated to be effective in field production of a number of susceptible crops. The active ingredient is Coniothyrium minitans, a fungus that parasitizes the sclerotia. It must be applied well in advance to symptom development (3 to 4 months) and/or can be applied to the soil and crop residue in fall following harvest. It can be applied both inside the high tunnel as well as in the outside surrounding area and is OMRI approved. See the label for additional application directions. Preventative applications of Endura for management of gray mold prior to flowering will also help manage timber rot.
Is there any difference in managing Sclerotinia in soil versus soilless media?
No, there is no difference in managing timber rot in field soil versus soilless media.