Improvements in Assays for Soil Populations, of Pythium ultimum and Macrophomina phaseolina

The method of Stanghellini and Hancock to enumerate Pythium ultimum Trow, populations in soil was modified by distributing 1 ml dilutions of soil containing the pathogen into 100–120, 2.5 mm diam. Wells made in 2% water agar in 9 × 15 mm Petri dishes. The modified method was more sensitive than the original method m which droplets or soil dilutions are placed on agar, it permitted the use of lower soil dilutions, and was rapidly performed. Sclerotia of Macrophomina phaseolina (Tassi) Goid. (=M. phaseoli Maubl.] Ashby) were recovered by flotation using 60% sucrose, collected by aspiration onto nylon filters, and surface-sterilized. The viable Sclerotia were enumerated by counting colonies growing on a selective medium. Efficiency of recovery was 95–97% of Sclerotia added to soil. Large numbers of samples could be processed rapidly by this method which utilized readily available materials, and gave higher counts than earlier methods, which involved only sieving or flotation.