Pathogenic and Saprophytic Growth Rates of Fusarium oxysporum f. sp. vasinfectum Interact to Affect Variation in Inoculum Density and Interannual Infection Risk.

Fusarium wilt of cotton caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) is a contemporary epidemic affecting cotton production in Far West Texas. The spatial distribution of soilborne FOV4 can be heterogeneous at small scales, and the factors that lead to this heterogeneity require investigation. Hypothetical causes include dissemination of spores through soils and variable saprophytic growth of the fungus. In the field, FOV4 DNA was quantified from soil during and after the cotton-growing season, and though the average amounts of DNA were not different between these time points, the variances of DNA across space were significantly different. Variability was higher when pathogenic growth of the fungus was expected owing to the presence of live cotton plants and lower when saprophytic growth was expected after cropping. In sterile-environment growth chamber experiments, the abundance of organic matter influenced the fungal vegetative growth rate and maximum amount as measured through quantitative PCR and the timing of the fungus' increasing its rate of spore production as measured through dilution plating. To investigate movement of spores in soils, spore mobility in experimental columns was quantified. Soil composition and organic matter abundance affected spore mobility, indicating that the timing of spore production relative to the availability of growth resources will affect the spatial spread of FOV4 and suggesting that soil properties affect the retention of conidia. The spatial spread of FOV4 through soil varies temporally and is affected by the shift between pathogenic and saprophytic growth modes of the fungus.

DNA-Based Quantification of Fusarium oxysporum f. sp. vasinfectum in Environmental Soils to Describe Spatial Variation in Inoculum Density.

Fusarium wilt of cotton, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. vasinfectum (FOV), occurs in regions of the United States where cotton (Gossypium spp.) is grown. Race 4 of this pathogen (FOV4) is especially aggressive, and does not require the co-occurrence of the root knot nematode (Meloidogyne incognita) to infect cotton. Its sudden appearance in far-west Texas in 2016 after many years of being restricted to California is of great concern, as is the threat of its continued spread through the cotton-producing regions of the United States. The aim of this research was to analyze the spatial variability of FOV4 inoculum density in the location where FOV4 is locally emerging, using quantitative and droplet digital PCR methods. Soil samples collected from a field with known FOV4 incidence in Fabens, Texas, were analyzed. Appreciable variation in inoculum density was found to occur at spatial scales smaller than the size of plots involved in cultivar trial research, and was spatially autocorrelated (Moran's I, Z = 17.73, P < 0.0001). These findings indicate that, for cultivar trials, accounting for the spatial distribution of inoculum, either by directly quantifying it or through the use of densely distributed calibration checks, is important to the interpretation of results.