The importance of Pythium in the production of container grown plants

Abstract

The major components of potting mixtures in New Zealand nurseries viz. soil, sand, pumice and peat were assayed for the occurrence of Pythium spp. using a selective medium. Pythium spp. were recovered from all but one of 23 soil samples and from 13 of 20 sand and pumice samples. No Pythium spp. were recovered from the upper 25cm of peat deposits at Waitakaruru, near Thames, or from the roots of plants growing in the area. Pythium spp. were recovered from a fenced-off area of cultivated field soil on each of 53 sampling dates over a period of 12 months. The populations ranged from 174-2203 viable propagules/g, and were found to be directly influenced by soil moisture but not by soil temperature. Pythium spp. were found to be frequently associated with diseased or decaying roots of container grown plants in 12 North Island nurseries. A list of the hosts from which species of Pythium were isolated is presented. In the course of the survey 16 species of Pythium were recovered. P. acanthicum, P. afertile, P. anandrum, P. aquatile, P. echinulatum, P. megalacanthum, P. middletonii, P. rostratum, P. splendens and heterothallic forms of Pythium are recorded for the first time in New Zealand. Sixteen species of Pythium, two atypical species, and nine heterothallic isolates of Pythium, were tested for their ability to infect the roots of tomato seedlings in a laboratory assay. Isolates of the same species, with the exception of P. megalacanthum were also tested in a glasshouse trial for capacity to cause damping-off of germinating seeds and seedlings of tomato c.v. 'Potentate', pea c.v. 'Victory Freezer' and Ipomoea violacea L. c.v. 'Heavenly Blue'. According to their capacity to infect seeds or seedling roots in soil, the Pythium spp. tested were classified as follows: (i) Highly pathogenic – P. acanthicum, P. debaryanum, P. irregulare, P. spinosum, P. splendens, P. ultimum, P. (?) ultimum. (ii) Moderately pathogenic - P. afertile, P. aquatile, P. middletonii P. monospermum. (iii) Mildly pathogenic – P. anandrum, P. (heterothallic isolates) Nos 1 and 4, P. torulosum. (iv) Not pathogenic – P. echinulatum, P. inflatum, P. rostratum, P. (?) rostratum. Pathogenicity of the Pythium spp. tested was similar on tomato seedlings in the laboratory assay and germinating tomato seeds in soil. P. acanthicum, P. debaryanum, P. irregulare, P. spinosum, P. splendens, P. ultimum and P. afertile were shown to cause root rot with associated symptoms of stunting, chlorosis and occasionally death of eight test crops which had developed beyond the seedling stage of growth. No significant differences in pathogenicity between the Pythium spp. were recorded. Control plants inoculated with Phytophthora spp. showed slightly more severe root rot symptoms. When the same seven species of Pythium were tested for capacity to cause root rot of nine-month-old seedlings of Pinus radiata at 15, 22 and 30°C, the first six species caused most damage at 30°C while P. afertile was most damaging at 15°C. Severity of infection by all seven species was least at 22°C. Pythium spp. were recovered from 11 of 13 silt and mud samples from Auckland water reservoirs using direct plating, filtering and baiting techniques. The same techniques failed to yield Pythium spp. from treatment stations or tap water. Pythium spp. were frequently recovered from rain-splash collected in beakers placed on cultivated ground, grassed areas or soiled asphalt. They were not recovered in rain-splash from clean asphalt or 30cm above soiled asphalt. Mycelial growth and sporangial germination of P. ultimum were completely inhibited in solutions of sodium hypochlorite containing 145 and 111.6 ppm Cl respectively. Swimming movements of zoospores and germination of encysted zoospores of P. afertile were completely inhibited in solutions containing 0.27 and 0.81 ppm Cl respectively.