Competition between two phenotypes of the nematophagous fungus ARF in infecting eggs of Heterodera glycines and effect of soil depth on parasitism by ARF.

The sterile nematophagous fungus ARF (Arkansas fungus) had been separated into two phenotypes based on morphological differences: those with compact sclerotium-like-structures (SLS) (ARF-C), and those with loose SLS (ARF-L). The ARF-L isolate TN14 had been more effective than the ARF-C isolate BG2 in suppressing nematode numbers in glasshouse studies, and the BG2 isolate had been more virulent in in vitro tests. No tests had been conducted on the effect of depth in the soil on the level of parasitism by ARF. The objectives of this study were to determine: i) which isolate is more competitive in field microplots by comparing isolation frequency of the two fungal strains from parasitised eggs of Heterodera glycines; and ii) whether parasitism by ARF varies with soil depth (0-10, 10-20, 20-30 cm deep). TN14 was isolated more frequently than BG2, irrespective of whether the original mixture of BG2 and TN14 was 50% TN14 + 50% BG2, 75% TN14 + 25% BG2, or 25% TN14 + 75% BG2. The percentage of parasitised eggs of H. glycines by ARF was different for the 0-10 cm soil layer vs the 10-20 cm soil layer, or the 0-10 cm soil layer vs the 20-30 cm soil layer. Parasitism by ARF was greatest at a soil depth of 10-20 cm. This would correspond with the level where the root concentration and numbers of cysts should be greatest. [ABSTRACT FROM AUTHOR]