Black Root Rot of Cotton

Abstract

Black root rot is a soil-borne fungal disease affecting cotton seedlings upon germination. Disease symptoms include stunted or slow growth early in the season compared to surrounding healthy plants. Diseased plants show blackening of the root and reduced number of lateral roots. The blackening is due to both necrosis (rotting) of the external layer of the root tissue (cortex) and the build-up of the fungal pathogen dark spores (reproduction bodies containing melanin) on the root surface. Previous laboratory studies indicated that 'T. basicola' could complete its life cycle on cotton within 72 hour post-inoculation, when spores (thin-walled endoconidia and thick-walled chlamydospores) appeared on the surface of the inoculated roots. The fungus can infect the centre of the root, causing a "black heart". Figure 1 below illustrates the black root rot disease cycle and shows the morphology of the fungal spores. The fungal pathogen causing black roo rot, 'Thielaviopsis basicola', is found in almost every inspected cotton farm in Australia and, while it does not kill the plant, it causes substantial loss of yield and severe cases might open the root up for infection by Pythium or Rhizoctonia. It has been considered a significant threat to cotton and other crops in Australia, especially in cooler areas and seasons. The symptomatic stunting of seedlings stays with the plant through to maturity, causing up to 40% yield loss, depending on the season, environmental conditions and cotton growing practices. When assessing for black root rot one should note that plants that are badly affected early in the season may not continue to show symptoms later in the season as the dead cells of the root cortex may slough off when growth resumes in warmer weather. However, the spores released into the soil increase the soil reserves of the pathogen. In following seasons, one may no longer observe patches of stunted growth as the fungal spores may have spread and the entire field might be infected.