Cause: A large number of soil-borne fungi, bacteria and nematodes are present within the upper few inches of soil. A majority of these under specific environmental conditions can cause plant disease. As we consider the soil-borne organisms that cause seedling diseases, we must remember that some of these microorganisms are beneficial for plant growth.
All plants have associated with their root systems certain groups of microorganisms. These organisms are activated by the presence of the plant roots and the substances released by them. Some remain active within the zone of the root influence while others are more active on the root surface. Others invade the root directly or through wounds and move into the cortical tissues or on into the vascular system. Obviously, the microorganisms obtain their nutrients from the plant. Some microorganisms release various substances (enzymes) which the plant may absorb and which may disrupt normal plant growth processes.
Symptoms: Diseases of the damping-off type occur during the early stages of plant growth especially when soils are cold. Immature seedlings are more susceptible than older plants. Three stages of growth during the susceptible stage have been described. First, the germinating seed becomes susceptible once it begins to swell and breaks, allowing potential damping-off organisms to gain entrance. Second, the new roots or shoots which emerge from the seed before it emerges from the soil may be attacked. Third, young seedlings may be damaged at or near the soil surface following emergence, causing them to fall over.
Control: Overwatering tends to encourage damping-off; only enough water should be added to seedlings to prevent wilting. Water in the morning and allow to dry before night. When seedlings begin to damp-off, they should be given as much light as possible. Sterilization of the soil before planting is one of the most satisfactory means of preventing loss of seedlings in garden areas. Small lots of soil can be baked for 3 to 4 hours in a 160 degrees F oven to kill the damping-off organisms.
Damping-off can also be controlled to a marked degree by seed treatments and soil drenches. Use fungicides at rates recommended on the label. Fungicides considered as good soil drenches include Captan, Thiram, Terraclor, and Benlate.
In many cases, seed treatment alone is sufficient to protect the young plants against damping-off disease.
A black velvety coating made up of the fungal strands is formed on the surface of leaves, twigs and fruit. If the honeydew is light, it may appear only in spots. As a general rule, the black fungus coating usually can be rubbed off easily from the surface of leaves, fruit or branches. With time, the fungus may dry-off, become flaky, and fall off. If for some reason the insect infestation decreases, the amount of sooty mold also will decrease. If no insects are present to cause a re-infestation, rains will usually wash off most of the sooty mold.
The fungi causing sooty mold are known to occur on citrus, oleander, gardenia, fig, crapemyrtle, azaleas, pittosporum and many other ornamental bushes and trees. Control can be obtained by applying insecticides that reduce insect populations. Using oil formulations as insecticides is effective, since oil gets rid of many of the insect pests and also softens the black fungus so it can be washed off easier by rain or other means.
Symptoms: Whitish fungal growth develops around the base of herbacious plants (and a few woody plants ) at the ground line. Small seed-like structures (sclerotia) are found with the fungal growth. They are white at first and later turn dark brown to black. Plants wilt and die suddenly after the fungus girdles the stem.
Disease development: Southern blight is especially destructive on crops such as tomato, beans, peas and peanuts. Many other plants including annual ornamentals are also susceptible. The fungus develops rapidly during hot weather when temperatures are over 85 degrees F. It grows on living and non-living organic matter and becomes most severe when dead leaves or other types of organic matter are present around the base of the plant. This permits the fungus to build up momentum by utilizing energy from the decaying organic matter and rapidly kill the host plant. The fungus develop rapidly when summer rains occur after a drought. This stimulates germination of the sclerotia (seed-like structures) and furnishes needed moisture for fungal growth. If the fungus finds ample organic matter and host plants, a large supply of sclerotia are produced for next year. These structures have a hard thick covering that resists weathering.
Control: Southern blight can be controlled with cultural and chemical techniques. Burying crop residue deep enough to prevent its being brought back up with land preparation and cultivation practices will remove the fungus and its food source. The fungus requires oxygen for development and deep burial reduces its activity. Keeping fallen leaves or other organic matter from the base of the plant is helpful. Using foliage fungicides to prevent foliage diseases will help keep leaves on the plant and off the ground. Fungicides may also be applied to the soil on certain crops. This will inhibit development of the fungus. Planting on a slightly raised bed help reduce damage on some crops.
In many cases, the disease is a continuation of seedling blight. Spots of various sizes occur on the stem, at or near the soil level and on the roots. These spots may vary in color from gray, brown, black, or even bright red. Frequently, these fungi cause the tips of fibrous roots to decay. Wilting, dieback, and poor vigor are common symptoms. Plants are often predisposed to infection by poorly drained soils, crowding, mechanical injury, over-watering, improper balance of plant nutrients or other factors that affect plant growth. Containerized plants often suffer root rot even though factors other than fungi may be the primary cause. Roots on potted plants can be checked for vigor by carefully removing the pot when the container is inverted and the plant is supported at the base with one hand. Dark decaying roots on the outside of the soil mass indicate a root rot condition may exist.
Control is difficult because once symptoms are observed, damage to the stem or roots is usually severe. For small flowerbeds and potted plants, use a soil drench of a recommended fungicide. A fungicide could also be mixed with soil prior to planting as suggested by the manufacturer. Allow excessively wet soils to dry. Always avoid throwing soil to stems when cultivating and avoid crowding plants in seedbeds or other areas. When transplanting or repotting, place plants at the same soil depth. Do not mulch heavily with partially decomposed organic matter. In the field, cover crops should be plowed under early to allow complete decomposition before a susceptible crop is planted.
Nematodes may cause symptoms similar to stem and root rots. See sections on Nematodes other than Root Knot, Southern Blight, Seedling Blight, Mushroom Root Rot, and Cotton Root Rot. All of these can cause stem and root rot type symptoms.