Growth of Phymatotrichopsis omnivora in Flutriafol Treated Field Soils

Flutriafol (Topguard) fungicide has shown efficacy for control of cotton root rot, caused by the soil borne fungus Phymatotrichopsis omnivora, in field trials in Arizona, but results have been variable. To determine activity of flutriafol against P. omnivora in Arizona soils, a laboratory bioassay was developed to determine growth of P. omnivora in soils sampled from experimental field plots treated with flutriafol or untreated at two different locations.  Flutriafol was applied at 32 oz/A in 4 gal water/A at planting as (1) a 5-inch wide T-band spray over the seed furrow, or (2) an in-furrow treatment utilizing a seed firming device (Schaffert Mfg.).  Fields were pre-irrigated, and seed was planted into moisture.  Fields were not irrigated again until after plant emergence.  Soils were sampled in the seed line using a two-inch auger at 0-4 and 4-8 inches after several irrigations. Soil samples were sieved through a #10 screen, and subsamples were placed in 100x25 mm Petri dishes to 5 mm depth and adjusted to 30% moisture (w/w).  One-cm cubes of sorghum seed colonized with P. omnivora were placed on soil in dishes and incubated at 27^o C for 7 days.  Growth and fungal strand development of P. omnivora was inhibited in all samples from flutriafol treated plots, but was not inhibited in soils from untreated plots. Results indicate that flutriafol moves downward in soil from the application site and is active in the top 8 inches of soil for at least 3 irrigations.  It was concluded that variability in disease control is not the result of loss of fungicidal activity in the upper root zone.