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Influence of Dinitroanaline Herbicides on Growth, Sporulation, and Infectivity of Four Phytophthora spp. Pathogenic to Deciduous Fruit Trees. W. F. Wilcox, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; Phytopathology 86:906-913. Accepted for publication 23 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-906.

Crown rot of potted Mahaleb cherry seedlings, caused by Phytophthora cryptogea, P. cambivora, and P. megasperma, was virtually prevented by applying the herbicide oryzalin at either of two nonphytotoxic rates (1.12 and 2.24 kg/ha) to a porous soil mix (1 volume of sandy loam:2 volumes of vermiculite) prior to infestation with the pathogens. In untreated pots, leaf-disk baits exposed during each of five to nine biweekly flooding periods were colonized at frequencies of 96 to 99, 49 to 95, and 13 to 47% by the respective pathogens, but only 0 to 7% of the baits were colonized in pots containing oryzalin-treated soil mix. At 2.24 kg/ha, pendimethalin similarly reduced both the incidence of disease caused by P. cambivora and its frequency of isolation from baits, but pendimethalin had little effect at 1.12 kg/ha. Likewise, pendimethalin had little effect on the incidence of crown rot caused by P. cryptogea when applied at either rate and only modestly reduced the frequency of its baiting. When tested in vitro at 0.06 to 4.0 g/ml, both herbicides significantly affected pathogen activities. At a concentration approximating that found in solutions of treated field soils (0.25 g/ml), oryzalin inhibited the formation of sporangia by P. cambivora, P. cryptogea, P. cactorum, and P. megasperma by 97, 89, 80, and 65%, respectively, whereas pendimethalin inhibited formation by 93, 49, 54, and 74%, respectively, at this concentration. When added to soil extract containing freshly released zoo-spores of P. cryptogea, P. cactorum, or P. megasperma, both herbicides significantly reduced the motile period, although pendimethalin generally provided the greater reduction at lower concentrations. Oryzalin was much more inhibitory than pendimethalin to mycelial growth of all four pathogens, although reductions were only modest (?20% relative to the checks) at concentrations ≤0.25 g/ml.