|Okra seedlings affected by damping-off
|(c) A.A. Seif & A.M. Varela, icipe
Damping-off is caused by fungus and it usually occurs in small patches at various places in the seedbeds or field. The disease spots often increase from day to day until the seedlings harden. Seedlings are extremely susceptible for about two weeks after emergence. As the stem hardens and increases in size, the injury no longer occurs. Some seedlings are not killed at once, but the roots are severely damaged and the stem is girdled at the ground level. Such plants remain stunted and often do not survive transplanting.
The injury from damping-off fungi is of two types:
- Pre-emergence damping off consists of a decay of the germinating seed or death of the seedling before it can push through the soil. This injury is a common cause of poor stands, which are often attributed to inferior quality of the seed or the untreated seeds. Pythium spp. and Phytophthora spp. cause seed decay.
- Post-emergence damping-off which occurs after the seedlings have emerged from the soil but while still small and tender. The roots may be killed, and affected plants show water soaking and shrivelling of the stems at the ground level; they soon fall over and die. Post-emergence damping-off is mostly caused byRhizoctonia spp.
The fungal disease caused by Rhizoctonia solani has a very wide host range, infecting plant species belonging to 32 families, and 20 weed species from 11 families.
In crucifers, this fungus causes damping-off and wire stem of seedlings in the seedbed; bottom rot and head rot in the field; and storage and root rot of horseradish, radish, rutabaga and turnip.
Seeds can decay in cold wet soils and stems can become light brown and water-soaked near the soil line. Such seedlings wilt, topple and die. Wet soils and temperatures at or above 24degC favour disease development.
Wire stem in cabbage:
This is the most common and destructive phase of the disease. The stem above and below the soil line shrivels and darkens, and outer tissues come off leaving a dark wiry and woody inner stem. Such plants do not fall over, but they have an unhealthy stunted appearance. Some may die, but most survive and do poorly when transplanted to the field. When moisture is adequate, plants may produce a small poor-quality head.
Bottom rot in cabbage:
The disease occurs in mid-season as a carry-over from wire stem seedlings and from new infections that occur when outer leaves come in contact with moist infested soil. Lower leaves wilt, decay and turn black, but do not drop off. Some plants may recover and produce heads, but usually bottom rot develops into head rot.
Head rot in cabbage:
A firm to slimy dark decay at the base of outer leaves and in cabbage heads develops during the period between head formation and maturity. The fungus grows up to main stem, passing between the leaf petioles. Foliage leaves die and drop off, thus exposing the stem beneath the head. Over the whole head surface, brown fungus mycelia and tiny brown resting fungal bodies (sclerotia) may develop and be visible over the head surface. Secondary rot bacteria usually invade the diseased tissue and turn the head into a slimy foul-smelling mass.
It is usually dark brown, sunken and spongy. Infected tissues easily separate from advancing edges of the rot. A white to brown surface mould and irregular brown sclerotia distinguish this rot form other root rots. It mainly affects horseradish, radish, rutabaga and turnip.
Affected plant stages
Heading stage (in cabbage), post-harvest (in cabbage), pre-emergence, seedling stage and vegetative growing stage.
Affected plant parts
Leaves, roots, seeds, stems and whole plant.
Leaves: lesions; abnormal colours; abnormal forms; wilting; fungal growth.
Seeds: rot; discolourations.
Stems: external discolouration; canker; abnormal growth; mycelium visible.
Whole plant: plant death; dieback; damping-off.