Effects of Plant Density on Progress of Phymatotrichum Root Rot in Cotton

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Abstract
An experiment was conducted at the Blackland Research Center, Temple, TX [Texas, USA], to determine the effects of plant density on the increase of Phymatotrichum root rot, caused by Phymatotrichum omnivorum, in cotton. Disease was assessed on 15 occasions beginnng 18 June and ending 20 August 1985 in 32 plots (7 .times. 8 m) of four replicates of eight plot density treatments: two between-row spacings, 69 and 138 cm, and four within-row plant densities, 1, 5, 9, and 13 plants per meter. A logistic model was fitted to disease progress data in each plot. Between-row spacing had no effect on the rate or other parameters of disease progress. A within-row density of five plants per meter reduced initial disease incidence (y0) and delayed the buildup of Phymatotrichum root rot compared with nine and 13 plants per meter but had no effect on asymptotic disease incidence (M) or area under the disease progress curve (Z). The rate of disease progress (r) was higher in plots with five plants per meter because of the initial delay in Phymatotrichum root rot progress in these plots. A within-row density of one plant per meter reduced y0, M, and Z. Initial disease incidence and Z increased linearly with plot density (overall plant density within plots) and within-row density. The asymptotic disease value (M) was exponentially related to plot density and indicated a threshold plant density of about one or two plants per meter above which M was unaffected. There were no consistent relationships between r and plot density or within-row density. The between-plant spacing that inhibited plant-to-plant spread of P. omnivorum in the experiment was estimated to be between 0.20 and 0.69 cm.

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