INFLUENCE OF DINITROANALINE HERBICIDES ON GROWTH, SPORULATION, AND INFECTIVITY OF 4 PHYTOPHTHORA SPP PATHOGENIC TO DECIDUOUS FRUIT-TREES

Crown rot of potted Mahaleb cherry seedlings, caused by Phytophthora c ryptogea, 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 vo lumes of vermiculite) prior to infestation with the pathogens. In untr eated pots, leaf-disk baits exposed during each of five to nine biweek ly flooding periods were colonized at frequencies of 96 to 99, 49 to 9 5, 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 dise ase caused by P. cambivora and its frequency of isolation from baits, but pendimethalin had little effect at 1.12 kg/ha. Likewise, pendimeth alin had little effect on the incidence of crown rot caused by P. cryp togea when applied at either rate and only modestly reduced the freque ncy of its baiting. When tested in vitro at 0.06 to 4.0 mu g/ml, both herbicides significantly affected pathogen activities. At a concentrat ion approximating that found in solutions of treated field soils (0.25 mu g/ml), oryzalin inhibited the formation of sporangia by P. cambivo ra, 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 e xtract containing freshly released zoospores of P. cryptogea, P. cacto rum, or P. megasperma, both herbicides significantly reduced the motil e period, although pendimethalin generally provided the greater reduct ion at lower concentrations. Oryzalin was much more inhibitory than pe ndimethalin to mycelial growth of all four pathogens, although reducti ons were only modest (less than or equal to 20% relative to the checks ) at concentrations less than or equal to 0.25 mu g/ml.