1Department of Plant Pathology and Microbiology, Texas Agricultural Experiment Station, Texas A&M University, College Station, TX 77843, USA; 2Ecological Phytopathology Group, Agricultural University, POB 8025, 6700EE Wageningen, The Netherlands

Background and objectives
Soil water potential can influence root disease development by direct effects on growth, sporulation, dispersal, germination and survival of fungal pathogens, or by predisposing host roots to infection or subsequent colonization. We have assessed the effects of soil water potential on fungal activity and disease development by examining components of the life cycle of the pathogen Phymatotrichopsis omnivora (Dug.) Henneb., which causes root rot in many economic crops in south-western USA and northern Mexico [1]. Analysis of recorded epidemics over a 14-year period had previously indicated that disease incidence in cotton at harvest was directly related to cumulative precipitation during the growing season. Our observations of cotton plants inoculated with sclerotia of the pathogen and transplanted into field microplots indicated that reduced water potentials delayed symptom development, which was most rapid at high water potentials and high soil temperatures. Sclerotia were formed most abundantly in association with diseased plants in soils near saturation. A more detailed study was made to assess direct effects of soil water potential on strand growth and scierotial formation.

Materials and methods
Root chambers were placed in soil-air temperature tanks with minirhizotrons, constructed to allow recording using a colour microvideo camera, inserted into the chambers. In situ effects of constant and cycling soil water potential treatments on strand growth and sclerotium formation were monitored in a series of experiments.

Results and conclusions

Strand growth of P. ;omnivora was greater in soil subjected to a regime of decreasing soil water potential compared with a continuously wet treatment; intermediate cycling treatments in the range -0.5 to -0.9 ;MPa resulted in less strand growth than either the drying or continuously wet treatments. A simple model was proposed to account for the pattern of strand growth observed. Sclerotia were recovered in greatest numbers where high water potentials (>-0.2 ;MPa) occurred at the time of plant death. The proliferation of hyphae and strands observed upon germination of sclerotia supported the view that major epidemic outbreaks result from plant-to-plant spread of the fungus. Minirhizotrons enabled the repeated measurement of hyphal length and growth rate and gave an estimate of hyphal turnover. Limitations of the technique for this purpose include the small sample size, the time-consuming conversions of the video recordings and the physical differences between the soil-rhizotron interface and the bulk soil. Nonetheless, the technique should have wide applicability for studying water-potential effects on other sclerotia-forming pathogens.

1. Kenerley CM, Jeger MJ, 1992. In: Hillocks RJ, ed. Cotton Diseases. Wallingford: CAB International, pp. 161-190.
2. Kenerley CM, Jeger MJ, 1998. Plant Pathology 47 (in press).