Spatial Dynamics of Phymatotrichum Root Rot in Row Crops in the Blackland Region of North Central Texas

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Abstract
The spatial dynamics of Phymatotrichum root rot [Phymatotrichum omnivorum] were mapped in field plots of cotton [Gossypium hirsutum] and soybean [Glycine max] at Temple and cotton only at Brandon in the Blackland region of north central Texas in 1984. First symptoms of Phymatotichum root rot were observed in mid-June, and assessments were made until mid-August. Analysis of the spatial dynamics indicated that the expansion of runs (sequences of diseased plants within rows) was a major component of increase. The spatial patterns of increase were consistent with plant-to-plant spread by hyphal strands in addition to a time-dependent symptom expression corresponding to an underlying distribution of sclerotia. Various analysis were used to describe and quantify the spatial patterns and dynamics of Phymatotrichum root rot. Spatial autocorrelation analyses were used to test the hypotheses of nonrandom spatial patterns of diseased plant within and across rows. The patterns observed indicated that plant-to-plant spread wthin rows was more important than spread across rows. The influence of neighboring 1-m row segments of plants, both continguous (within rows) and adjacent (across rows), was evaluated by calculating the relative frequencies that a disease-free segment contained at least one diseased plant after a 7-day interval. A linear regression model was derived to predict the relative frequency of transition in terms of the status (diseased or healthy) of its nearest neighbors. The regression coefficients indicated that the influence of contiguous neighbors was more important than that of adjacent neighbors. The relative frequencies that a run of a given length class extended to a longer class during a 7-day interval were calculated and found to be inversely related to the difference in length between length classes.