Subterranean Clover Foliage Fungi as Root Pathogens.

Abstract

38 medicaginis malbr. and Roum. are commonly isolated from cotyledons, leaves and petioles of subterranean clover (Trifolium subterraneum L.); (M.J. Barbetti, unpublished data). L. trifolii and P, medicaginis have been isolated from the foliage of almost all subterranean clover sampled from the lower southwest of Western Australia (M.J. Barbetti, unpublished data). Although common on subterranean clover foliage, F. eveneceum and M. verruca ria occur at a lower frequency than L. trifolii and P. medicaginis. Both F. avenaceum and P. medicaginis have been isolated from rotted subterranean clover roots (10, M.J. Barbetti, unpublished data). F. avenaceum is a serious root pathogen of subterranean clover in eastern Australia (4, 5), and is associated with some root rots of subterranean clover in Western Australia (9, 10). P. medicaginis has recently been shown to be a weak pathogen on subterranean clover in Western Australia (10). Overseas, M. verruca ria is known to cause root rot of red clover and alfalfa (6). L. tritotil has not been reported as a root pathogen. The present study investigated the pathogenicity of foliage isolates of each of the fungi described above to subterranean clover roots, For a single isolate of each fungus, inoculum was prepared by growing the fungus on moist sterile wheat for 3-4 weeks at room temperature. Sterile wheat was prepared by adding 150 ml of distilled water to 200 g wheat in a 1 I flask, soaking for 8 hr. then autoclaving at 100 kPa for 30 min on two consecutive days. The required amount of colonised wheat (0.5, 2.0 or 5.0% w/w) was thoroughly mixed with pasteurized potting mix (sawdust:sand, 50:50 v/ v mix, treated with aerated steam for 90 min at 60 C). Twenty seeds of the cultivar Yarloop (root rot susceptible) were placed on top of the inoculum/soil mix in a 10 em pot and covered with 1 em of pasteurized soil. Treatments were replicated six times in a fully randomised design. Pots were watered daily with de-ionized water. Tests were made in growth cabinets each with a 10.5 hr day, a 13.5 hr night, a light intensity of 35,000 lux, and set at day/night temperatures of 15/10,18/13 and 21/16 C, respectively. Four weeks after sowing plant survival was recorded, and plants were washed and rated for disease severity as previously described (2), viz. 1. Healthy tap root with 0 10% of tap root affected by root rot. 2. Moderate tap root rot 10 70% of tap root affected by root rot. 3. Severe tap root rot 70 100% of tap root affected by root rot. Plants were similarly rated for severity of damage to the lateral roots. The average percentage root disease index. based on the above disease ratings, was then calculated for both tap and lateral roots using the method described by McKinney (7). The foliage dry weight of each plant was also recorded. The test fungi were re-isolated from affected roots to confirm that they produced the disease symptoms. Results (Tables 1 and 2) show that each of the four fungi caused root disease. F. avenaceum was more severe than P. medicaginis, M. verruca ria, or L. tritolli. F. avenaceum and M. verruca ria caused most root rot at the two lower temperatures while L. triiolii caused most root rot at the two higher temperatures. All fungi reduced (p<0.05) plant weight at one or more inoculum rate/temperature combinations. The fungi had little or