Mycorrhizal colonisation, root browning and soil properties associated with a growth disorder of cotton in Australia

The symptoms of a growth disorder of cotton and associated properties of the soil were quantified. Data were collected from 35 sites across two irrigated fields that showed gradients in the severity of early season stunting of cotton. Ordination analysis of soil characteristics distinguished three groups of sites (A, B, and C) which corresponded to patterns of yield and early season growth. Group A and B soils had lower pH, finer texture and higher P, Zn, Mn and exchangeable Mg, K and Na than group C soils. Early season growth and arbuscular mycorrhizal colonisation of cotton at group A and B sites was much slower than at group C sites. Group B sites showed a recovery of yield later in the season while group A sites did not. Nutrient deficiencies, waterlogging, soil compaction, soil sodicity and Mn toxicity were unlikely causes of early season stunting, although soil Mn was a good predictor for the disorder. Thielaviopsis basicola, Verticillium dahliae and unidentified Chytridiomycetes were not associated with stunting and reduced yield, although other fungal pathogens may have been present. Root browning was a symptom of the disorder and suggests that pathogens, perhaps bacteria, play a causal role. This study showed that the growth disorder involved an interaction between cotton and the soil flora which was associated with heavy soil texture.