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Commercial pesticide formulations of triphenyltin hydroxide, benomyl plus triphenyltin hydroxide, and phosalone completely inhibited pollen germination of pecan [Carya illinoensis Wangenh C. Koch] when incorporated in in vitro germination media at one-fourth to one times the recommended rates. Scanning electron microscopic evaluations of spray effects on receptive stigmatic surfaces showed varying degrees of injury, ranging from minor surface wrinkling with triphenyltin hydroxide to severe collapse and degeneration of stigma papillae with phosalone treatments. Controlled pollinations 1 hour after pesticide sprays resulted in an inhibition of pollen germination and tube growth. Water sprays followed by pollination resulted in normal pollen adherence, hydration, and germination. Chemical names used: methyl[1-[(butylamino)carbonyl]-1H-benzimidazol-2-yl]carbamate (benomyl); S-[(6-chloro-2oxo-3-(2H)-benzoxazolyl)methyl] 0,0-diethyl phosphorodithioate (phosalone). Current recommendations for control of disease and insect pests in pecan include application of early season pesticides (Ellis et al., 1988). Preventive fungicidal sprays are advised to protect rapidly developing young foliage and to prevent establishment of disease in the orchard; insect and mite pests may necessitate early season application of insecticides. Thus, timing of pesticide applications frequently overlaps the pollination period. Adriance (1931), Hamilton (1942), and Woodroof et al. (1928) proposed that early season nut drop in pecan results from lack of pollination and hence absence of fertilization. This major period of drop, often referred to as May drop, coincides with the drop of bagged, unpollinated flowers. Effects of pesticides on pollen germination and growth in pecan are unknown. Inhibitory effects of pesticidal sprays during pollination would be of particular concern in areas where pollination and fertilization are limiting factors in fruit production. Previous reports on the negative effect of agricultural pesticides on pollen germination and growth in fruit crops include observations with pear and apple (Church and Williams, 1977; for publication 30 Oct. 1989. The cost of publishing this paper was in part by the payment of page charges. Under postal regulations, this refore must be hereby marked advertisement solely to indicate this Legge and Williams, 1975; Marcucci et al., 1983; Marcucci and Filiti, 1984), blueberry (Bristow, 1981; Bristow and Windom, 1987), raspberry (Redalen, 1980), and cranberry (Bristow and Shawa, 1981; Shawa et al., 1966). However, we found no studies that determined how, or if, spray applications affect pollination in pecan. Meaningful extrapolations from the other crops to pecan culture are difficult due to differences in chemicals evaluated and divergences in reproductive strategy and morphology. The majority of studies have evaluated pesticide effects on pollen germination using in vitro germination tests; relatively few have addressed the pollination response using in vivo systems. We found no reports on pesticide effects on stigma morphology and pollen-stigma interactions at the ultrastructural level. The objectives of this study were to evaluate the effects of selected pesticides on stigma receptivity and structure, pollen germination, and pollen tube growth, using in vitro germination tests and in vivo observations at the electron microscopic level. Materials and Methods In vitro germination tests. The pesticide combinations evaluated as commercial formulations and their recommended rates were: triphenyltin hydroxide (Du-Ter, Flowable 30, Griffin Corp., J. Amer. Soc. Hort. Sci. 115(4):656-661. 1990. Fig. 1. Scanning electron micrographs of the stigmatic surface of pecan distillate flowers. (A) Papillate cells covered with a surface exudate of electron-dense and -translucent areas. (B) Stigmatic surface after treatment with a water spray showing papillate cell separation and accumulation at cell interstices. (C) Exudate is retained on water-sprayed stigmatic surface. Valdosta, Ga.) 1.9 ml·liter water; Benomyl (Benlate, wettable powder, Dupont De Nemours, Wilmington, Del.) 600 mg·liter -1 water plus triphenyltin hydroxide 0.95 ml·liter water; and phosalone (Zolone, Rhone-Poulenc, Research Triangle Park, N.C.) 5 ml·liter water. Benomyl was evaluated in conjunction with triphenyltin hydroxide due to the recommendation that it not be applied alone (Ellis et al., 1988). Chemicals selected were widely used in pecan early season spray applications. For in vitro germination tests, pesticides were incorporated at onefourth, one-half, three-fourths, and one times the recommended rates into a-germination medium containing sucrose, B, and agar (Wetzstein and Sparks, 1985). Catkins from ‘Curtis’ pecan trees were collected the afternoon before use and allowed to dehisce overnight in the laboratory. Pollen was collected and hydrated for 30 min in a humidified chamber before inoculation. Germination was evaluated 8 to 12 hr later; 1,000 pollen grains per treatment were counted. Pollen was considered germinated if tubes grew longer than the pollen diameter. Stigma morphology and pollen-stigma interactions. Distillate inflorescences were bagged (Gerhold, 1968) on 15-year-old ‘Stuart’ trees before receptivity to avoid open pollination. At receptivity, about 2 weeks later, l-year-old branches with attached shoots were removed. Their bases were put immediately in water. For laboratory observations of pesticide effects on the stigmatic surface, inflorescences were sprayed to runoff by use of a hand sprayer and with the same pesticides as used for in vitro germination tests at recommended rates. Distilled water sprays served as controls. Stigmas were examined at 2 to 4, 6 to 8, and 12 to 14 hr after treatment. For microscopic observations, stigmas were excised, attached to aluminum stubs, and observed fresh and uncoated in a scanning electron microscope (Phillips 505 SEM); three to four flowers were observed for each treatment and time. For evaluations of pollen–stigma responses, pesticideand water-treated flowers were hand pollinated 1 hr after spraying. Flowers were observed 4 to 5 hr after pollination, as described for microscopic study of the stigmatic surface. Flowers were also observed microscopically for browning 14 to 16 hr after spraying.

Stigmatic Surface Degeneration and Inhibition of Pollen Germination with Selected Pesticidal Sprays during Receptivity in Pecan