Seed Rot and Seedling Blights(fungi and bacteria – several genera): A great deal of damage occurs to cultivated castor in the seedling stage. Seed-borne diseases were once considered uncommon, but at least ten parasitic fungi have been isolated from castor seed. Many of these penetrate the testa making disinfection difficult. Many of these fungi attack irrigated plantings of castor at any stage of growth, though most damage is on young plants.
After germination, seedlings are susceptible to a number of root and stem rot organisms, which become more dangerous in wet soils. The most common of these are species of Fusarium, Rhizoctonia and Sclerotium, all major pathogens of seedlings and capable of decimating young plants. The incidence of seedling wilt can be related to temperature, maturity of the seed planted and variety. Plants infected as seedlings may be predisposed to later fungal infection. Stunting accompanied by a black rot in the tap root, and elongated, brown lesions on the hypocotyl was caused by Thielaviopsis basicola. The use of a fungicide seed treatment is recommended for areas where seedbed temperatures may be lowered after planting, where the incidence of soil-borne pathogens is known to be high, and for irrigated plantings. Where the specific organism has not been identified in an area, suitable seed treatment may be obtained by following cotton recommendations, providing the chemicals have been cleared for the castor crop.
Charcoal Rot (fungus – Macrophomina phaseolina): This is a widespread and frequently serious disease of many crops. When intercropping and rotation are practiced, the disease becomes more damaging when all the crops are susceptible. Charcoal rot develops rapidly on crops under moisture stress after flowering and seedset. Temperatures between 77 and 95 degrees F favor rapid growth of the fungus. The disease causes blackening of the stem near the soil line, followed by premature death.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Castor is very susceptible and should not be planted in soils where the fungus is established. Cultural practices as recommended for root rot control of cotton may be effective.
Leaf Spot (fungus – Cercospora ricinella): Light brown, generally circular interveinal spots with margins of concentric rings, are produced. With age, the center of the spot changes to light gray.
Leaf Spot (fungus – Alternaria recini): Light brown, generally circular spots appear on leaves that are larger than Cercospora, which tend to become angular with age. A gray-green spore mass may sometimes be seen.
Bacterial Leaf Spot (bacterium – Xanthomonas ricinicola): Numerous, irregular, small, brown watersoaked spots occur on leaves, followed by premature defoliation. Spots gradually turn black with dried sections of leaf tissue disintegrating and falling from leaves. Racemes are attacked under humid conditions. Serious losses may occur under humid conditions.
Bacterial Leaf Rust (bacteria – Pseudomonassp.): Leaves dry up, turn black and fall. Branches also turn black, and stems may be affected, in which case the plants usually die.
Gray Mold (fungus – Botrytis ricini): The entire group of flowers is attacked and converted to a prominent wooly mass of fungal growth. Also affects leaves and stems by infection from racemes. First symptoms are small, blackish spots from which drops of yellow may exude. Fungal threads which grow from these spots spread the infection and produce the characteristic wooly appearance of the inflorescence.
Capsule Molds (fungi – Alternaria sp., Penicillium sp., and Fusarium sp.): Capsules are attacked at an early stage of development. Capsules have distinctive bluish color in early stages. Color may become darker or black in later stages of development.