Survival of Phytophthora colocasiae in Field Soil at Various Temperatures and Water Matric Potentials

A large number of the zoosporangia of Phytophthora colocasiae incorporated into moist soils germinated by zoospore discharge, and/or lysed in the soil during the first 5 days of incubation, decreasing the initial number of colony-forming units from 1 × 104 to 1 × 102 per g of soil in all treatments. Eighteen days after incorporation, the viable zoosporangia present in moist soils had thickened their cell walls and germinated only directly, often producing smaller zoosporangia. A few thick-walled chlamydospores were observed, and they germinated only directly. Zoosporangia in soils at -1,500 J/kg matric potential survived longer than 107 days, and the amount of viable Zoosporangia present at that time was approximately 0.1 × 102 CFU/g of soil. Apparently the great majority of the thin-walled zoosporangia produced on V8 agar, when incorporated into moist soil, germinated indirectly in the first 5 days of incubation. Zoosporangia that did not germinate became resting zoosporangia by increasing their wall thickness or by producing chlamydospores. These enabled the pathogen to survive in soil at -1,500 J/kg matric potential for more than 3 months. However, in the absence of the host, the pathogen is predicted to survive less than 1 year due to its lack of saprophytic ability to colonize nonhost tissues.

Reassessment of the Role of Saprophytic Activity in the Ecology of Thielaviopsis basicola

The ability of Thielaviopsis basicola to survive saprophytically in soil was investigated using root tissue from susceptible hosts as organic substrates. Inoculum densities were lower in soils amended with root tissue than in nonamended controls after 2 and 4 weeks of incubation. The greatest decrease occurred in soils containing the highest concentration of root tissue or in soils in which root tissue included the soluble components of the living root. Reproduction by T. basicola also was examined in axenic media containing either killed root pieces or various carbohydrates as the sole carbohydrate source. T. basicola utilized killed root tissue as a carbohydrate source in axenic media, particularly in cultures in which root tissue included the soluble components. Enzymatic activities of T. basicola, however, did not result in maceration of the root tissue. T. basicola utilized sucrose and cellobiose, but did not utilize structural carbohydrates such as cellulose, hemicellulose, or pectin. Based on the absence of significant saprophytic ability, T. basicola should be classified ecologically as an obligate parasite.

Distribution in Australia and host plant specificity of Phomopsis emicis, a stem blight pathogen of Emex australis

The stem blight pathogen, Phomopsis emicis, is widespread on Emex australis throughout those regions of southern Australia with a Mediterranean climate. Host specificity tests showed that P. emicis was able to cause leaf lesions or stem collapse on five closely related species, E. australis, E. spinosa, Rumex alcockii, R. dumosus and R. pulcher, in the Polygonaceae. Two of these species, R. alcockii and R. dumosus, are native to Australia and the others are introduced weeds. P. emicis was able to colonize the wounded or senescent tissue of several other plant species, thereby demonstrating its facultative saprophytic ability. There was evidence that P. emicis was able to survive as an endophyte in Muehlenbeckia adpressa and Polygonum aviculare.