Streptomycin Applications to Control Walnut Blight Disease Can Prevent Fertilization and Increase Fruit Drop

We investigated the basis for fruit drop in walnut (Juglans regia L.) following bloom period applications of streptomycin as a potential control treatment for walnut blight, a bacterial disease incited by Xanthomonas campestris pv. juglandis (Pierce) Dye. Experiments were conducted on streptomycin-treated field plots of ‘Vina’ walnut. Four streptomycin treatments were applied at different times relative to anthesis. Fruit from all treatments grew similarly for four weeks following anthesis when high levels of fruit abscission began to occur in the treatment sprayed during the bloom period. Microscopy revealed that in this treatment ovules failed to develop normally, and neither embryo nor endosperm developed. The pattern of fruit development and timing of fruit drop following streptomycin treatment at bloom is similar in all ways to that of unpollinated walnut flowers where growth appears normal until abscission occurs 3 to 5 weeks after anthesis. Pollen germination and pollen tube growth were inhibited in the bloom-period treatments. Pollen germination in vitro was not affected by addition of streptomycin to a germination medium. If streptomycin were to be used in a walnut blight control program, application timed to coincide with the period of pistillate bloom and pistillate flower receptivity should be avoided. 50% ethanol, 5% acetic acid (FAA). The ovaries were dehydrated in a tert-butyl alcohol series and embedded in Paraplast Plus (Sherwood Medical, St. Louis). Ovaries were sectioned at a nominal thickness of 8 μm and stained with safranin O-fast green FCF. The stigma-style portions of flowers from the prebloom, bloom-period and control treatments were softened, gently squashed on slides, stained with alkaline aniline blue, and observed with a fluorescence microscope. Number of pollen tubes present in the stigma and at the base of the style was recorded for 20 flowers for each treatment. Pollen of walnut cultivars Hartley and Vina were germinated in vitro using the medium of Luza and Polito (1987) supplemented with 400, 200, 100, 10, and 0 mg·L streptomycin added to the medium after it had cooled to below 50 °C. Three replicates were made for each cultivar at each concentration. Pollen was collected from catkins that were allowed to dehisce overnight. The pollen was hydrated in a saturated environment for 120 min, then incubated on the medium for 3 h after which the plates were sprayed with FAA to halt pollen tube growth. Germination was determined for 100 pollen grains from each replicate. Pollen was considered to have germinated when pollen tube elongation was at least twice the diameter of the pollen grain. Pollen tube growth was not measured directly, but a visual estimation of comparative pollen tube elongation was made.