Being an Extension agent, I have to say that one of the most important cultural practices that growers need to follow is crop rotation in order to minimize the buildup of disease organisms persisting in the soil. One particular case involving the failure to practice crop rotation comes to mind: A fresh market tomato grower had a field that was located next to a main road that was easily accessible to his packing house and the general public.
Because of this, he would plant tomatoes year after year in the same field. After a few years, his yields started to decrease dramatically and he couldn’t understand why. After sending plant samples to the plant disease clinic, it was confirmed the tomatoes were infected with Southern blight and also internal pith necrosis.
Southern blight is caused by a soil-borne fungus. This pathogen survives adverse conditions as sclerotia (fungal spores) in the soil and as mycelium (white, thread-like fungal strands) on decomposing plant residue. Sclerotia can persist in upper soil layers for many years. The disease tends to be more prevalent in sandy or sandy loam soil types and also in soils with high organic matter that has not decomposed. Conditions favoring disease development include high moisture levels, both in soil and under plant canopies, and at relatively high temperatures (81 – 95º F).
On tomatoes and peppers, the most common symptom occurs on the lower stem where it is in contact with the soil. Initially, a brown to black lesion usually develops on the stem near the soil line. The lesion will develop rapidly and can completely girdle the stem, which will cause a sudden and permanent wilt of all above-ground parts. Young plants may fall over at the soil line.
Under moist conditions, white mycelium mats will typically develop on stem lesions, becoming visible two to four days after infection, and can sometimes extend several centimeters up the stem of the plants. After a few days, tan to reddish-brown “mustard seed” spherical sclerotia (1 – 2 mm in diameter) can appear on the mat of mycelia, about seven days after infection.
The fungus can easily infect fruit that are in contact with infested soil. Lesions will initially appear as sunken and slightly yellow areas that later become water-soaked and soft, with star-shaped spots. The fruit will collapse within three to four days and white mycelium and sclerotia can fill the lesion cavity.
Short distance dispersal of sclerotia occurs via running water, movement of infested soil and debris on cultivating tools and equipment, and growth of mycelia from infected plants to healthy plants. Long distance spread can occur via infected transplants, infested soil or contaminated seed mixtures.
Southern blight can survive and overwinter as sclerotia and on host debris in the soil for years. The fungus is highly saprophytic (living on dead plant tissue) and can produce mycelial growth on a variety of hosts. However, the fungus is generally restricted to the upper two to three inches of soil and will not survive at deeper depths.
For example, in most North Carolina soils, the fungus does not survive in significant numbers when a host plant is absent for two years or more. High temperatures (77 – 95º F), aerobic and moist conditions and acidic soil favor disease development and fungal growth. Germination of sclerotia occurs at a soil pH between 2.0 and 5.0 and is inhibited at a pH higher than 7.0.
Southern blight is also known as basal stem rot, Sclerotium blight, crown rot and white mold. The more common vegetables and legumes affected (besides tomato and pepper) include bean, cabbage, carrot, peanut, cucumber, eggplant, garlic, muskmelon, potato, pumpkin, radish, rhubarb, turnip and watermelon.
Disease management practices depend upon the crop and production system. Implement as many of the following as possible to limit the risk for infection and/or impact of disease: Avoid planting susceptible crops in sites with a history of southern blight; plant only pathogen-free plants; examine lower stems and roots of transplants for signs of the fungus (fungal mats and sclerotia) before planting; and practice crop rotation by rotating to a non-host or one less preferred by the fungus for three to four years if the site has a history of Southern blight.
Southern blight has a large and diversified host range which can make selection of a rotational crop challenging. Suggested non-host crops include corn, small grains, grain sorghum or pasture grasses. Ornamental grasses are also reportedly resistant to this disease. Many weeds can serve as hosts to Southern blight, so weed management during rotations or during fallow periods is essential to reduce survival of the fungal spores.
Practice good sanitation practices by removing and destroying infected plants outside and in high tunnels/greenhouses. Remove surrounding soil and plant debris when digging plants. Do not allow sclerotia to drop back onto the planting site or surrounding soil.
Use deep tillage to bury sclerotia and infected plant debris after plants are removed. Survival of sclerotia declines at depths greater than six inches. Without crop rotation, deep plowing is less effective since previously buried sclerotia may be returned to the upper layer of soil. Improve air circulation and plant tissue drying by increasing plant spacing. Moisture trapped within a dense plant canopy favors disease development.
Another method that can be used to reduce the number of sclerotia is soil solarization, a process that uses clear plastic sheeting and can be an effective means to reduce pathogen levels in high tunnels or greenhouse ground beds. The treated areas should to be exposed to full sunlight and a soil temperature of 110 – 125º F by covering with clear plastic for at least four to six weeks.
Fumigation with chemical soil treatments can also be used to reduce a number of soilborne pathogens, but can also reduce populations of beneficial soil microorganisms. In addition, small numbers of sclerotia can survive, requiring repeated fumigant applications.
The other disease, internal pith necrosis, is caused by a weak soil-borne bacterium which is usually not able to infect without having a combination of factors present that make it easier for the bacteria to cause disease.
Many tomato growers use black plastic mulch and drip irrigation coupled with nitrogen fertilization. This growing method provides an excellent environment for the disease to gain a foothold if certain other conditions are met. Optimal conditions for pith necrosis include cool night temperatures, high humidity, prolonged periods of cloud cover and excessive soil nitrogen levels. Symptoms of this disease include yellowing of young leaves and leaflets, wilting of shoots on the upper portion of the plant (top wilting) and split or collapsed stems that are associated with dry, brownish-gray lesions. Stems are often hollow, black in color or have a “rungs of a ladder” appearance or chambered pith when they are split open lengthwise. Stems can be swollen, forcing large clusters of adventitious roots forming along the stem and are gnarled in appearance.
Internal pith necrosis can occur on plants randomly throughout the field. Sometimes, part of a plant can be affected but the non-affected plant part will continue to grow normally. Internal pith necrosis disease progression can be slowed during warm and sunny periods and plants will often recover from the disease once fertility conditions are corrected. The disease can also be prevented by avoiding the use of excessive amounts of nitrogen with tomatoes, especially early in the season when nights are still cool, which favors the rapid growth of plants and the ability of the bacteria to gain easy entrance to the plant.
In summary, it is vital for growers to practice crop rotation to minimize the buildup of soil-borne diseases that are specific to certain vegetables, and to avoid overfertilization of nitrogen on tomatoes to reduce the chance of contracting internal pith necrosis.