Some studies on ectotrophic mycorrhizae and Phytophthora cinnamomi on Pinus radiata

Abstract

The distribution of Pinus radiata mycorrhizae throughout the Auckland area was studied, and the interactions between mycorrhizae and Phytophthora cinnamomi investigated. Techniques were developed for the isolation and culture of mycorrhizal fungi and for the synthesis of mycorrhizae under axenic conditions. Mycorrhizal symbionts were most successfully isolated onto agar media after individual mycorrhizae had been washed for more than 1hr. Alternatively, where bacterial contamination was especially severe, a selective medium amended with 50ppm tetracycline was used. Woodhill State Forest, Riverhead State Forest and shelterbelts throughout the Auckland region were examined with regard to mycorrhizal development, along with nursery and glasshouse-grown P. radiata seedlings. Isolations from mycorrhizae showed a mycorrhizal flora consisting mainly of Rhizopogon along with Thelephora, Suillus, Amanita and Tricholoma spp. and a group of unidentified basidiomycete isolates. Comparison with isolates from sporocarps showed that not all species commonly found fruiting beneath P. radiata could be considered mycorrhizal, and that not all species capable of mycorrhizal production actually formed mycorrhizae in the field. Differences in the mycorrhizal flora were also noted between and within different sites in Woodhill State Forest, Riverhead State Forest, shelterbelts, nurseries and the glasshouse. Host age proved very important in determining the relative proportions of the different symbionts studied. Examination of mycorrhizae in the field showed differences in the pattern of mycorrhizal distribution with depth between different sites, although, in general, most mycorrhizae were concentrated near the soil surface. Different sites, type and age of stand also differed in the percentage of root tips mycorrhizal and the percentage of mycorrhizae senescent. Distinct forest and shelterbelt situations could be recognised and, as with symbiont identity, host age proved an important determinant of mycorrhizal development. On the basis of mycorrhizal development, 5 distinct classes of P. radiata could thus be recognised:- 1) nursery seedlings, 2) 1-2 yr forest seedlings, 3) 10-15 yr forest stands, 4) >20-yr forest stands, 5) >20-yr shelterbelt stands. Mycorrhizal development was pronounced in nursery-grown seedlings. In the field, however, a maximum level of mycorrhizal development was not attained until 10-15 yr, after which mycorrhizal abundance declined with increasing age. Mycorrhizal development in shelterbelt situations was typically less extensive than in equivalent-aged forest stands. Mycorrhizae could also be separated into immature (I), mature (M) and senescent (S) categories. Together, these characteristics proved a useful means of describing the mycorrhizal status of any given site. Mature mycorrhizae were thus most abundant in 10-15 yr forest stands (class 3) above, with increasing proportions of immature and senescent mycorrhizae in younger and older stands, respectively. The involvement of soil structure, temperature, soil fertility, pH and soil moisture in mycorrhizal development was also investigated. Mycorrhizal development was positively correlated with such aspects of soil structure as macroporosity and litter accumulation. Comparisons were made between Riverhead clay soils, Woodhill sands and shelterbelt pasture soils. Soil structure and litter deposition were typically observed to improve with host age. With regard to temperature, most symbiont spp. tested showed relatively high optima at 20-25 C. Pisolithus tinctorius and Thelephora terrestris showed especially high temperature optima, apparently connected with their limited field distribution. A comparison of Woodhill, Riverhead and shelterbelt sites with regard to soil fertility showed mycorrhizal development to be inversely related to nitrate availability, but directly proportional to phosphate availability and soil acidity. In Riverhead State Forest mycorrhizal development was typically limited by low phosphate availability while mycorrhizal development in many shelterbelt sites was restricted by high soil nitrate concentrations and high pH. Soil moisture also proved capable of affecting mycorrhizal development with both symbiont growth in the soil and the production of new mycorrhizae inversely proportional to soil moisture content. Waterlogging was thus especially disadvantageous for mycorrhizal development. Both waterlogging and drought induced higher-than-normal levels of intracellular penetration. The overall effect of the above factors could be seen reflected in the seasonal patterns of mycorrhizal development encountered – especially with regard to waterlogging, drought and the balance between mycorrhizal initiation and host root growth. Maximum mycorrhizal development and abundance was usually found in autumn, but this varied between different sites where climatic variation was either more or less extreme. The distribution of P. cinnamomi throughout the Auckland area was also investigated. Low, medium and high-risk sites could be distinguished on the basis of P. cinnamomi potential disease index, mycorrhizal type and abundance, stand type and age and the incidence of littleleaf symptoms. Most infection by P. cinnamomi was found to occur in late summer-autumn. Examination of infection by zoospores in root inoculation cylinders showed I and S category mycorrhizae to be especially susceptible. In contrast, M category mycorrhizae proved up to 100% resistant to infection, (successful infection by P. cinnamomi only occurred where mantle or Hartig net development was incomplete or disrupted). Sites with an especially low proportion of M category mycorrhizae (e.g. shelterbelts) proved especially vulnerable to infection by P. cinnamomi. No positive evidence was found for the involvement of antibiotics, host exudates, induced inhibitors or antagonistic rhizosphere populations in the defense of mycorrhizae against infection by P. cinnamomi.