Effectiveness of Postharvest Fungicides for the Control of Citrus Fruit Decays

Thiabendazole and benomyl were the most effective fungicides for the control of Diplodia natalensis Pole Evans stem-end rot, a major decay of early degreened fruit. This decay was also significantly reduced by imazalil and guazatine, but the level of control was not as consistent as that obtained with thiabendazole or benomyl. Control of Dip lodia stem-end rot was not obtained with DF-1OO or potas sium sorbate. Sour rot, caused by Geotrichum candidum Lk. ex Pers., was controlled with guazatine and etaconazole but not with DF-1OO, Purogene, fenpropimorph or potassium sor bate. Green mold, caused by Penicillium digitatum Sacc, was effectively controlled with imazalil, guazatine and thiabendazole. However, efficacy of thiabendazole was greatly reduced in the presence of strains of P. digitatum with resistance to the fungicide. Potassium sorbate and DF100 significantly reduced green mold only in some tests, but even then they were not as effective as imazalil or guazatine. Purogene did not control green mold and at high rates it significantly increased the decay. Of the fungicides tested, benomyl, thiabendazole, imazalil, and potassium sorbate are presently labeled and approved for application to citrus fruits to control post harvest decays. Appreciation is expressed to M. T. Davis for technical assistance. 208 Postharvest decays frequently cause extensive losses of Florida citrus fruit before or after it reaches the consumer. A significant proportion of this loss can be prevented by applying effective fungicides to the fruit soon after har vest. A program to evaluate, identify, and aid in the regis tration of effective fungicides for decay control in citrus has been pursued at the Lake Alfred Center for many years. Efforts to develop new fungicides are necessary for several reasons. Improvements in efficacy, safety and ex pense may be realized. Use of registered fungicides can be discontinued at any time due to unforeseen health or en vironmental hazards. Furthermore, some pathogens, par ticularly Penicilliumy can develop resistance to fungicides causing them to be ineffective. The purpose of this report is to describe recent studies where the effectiveness of certain fungicides was evaluated for the control of 3 major decays of Florida citrus fruit, namely Diplodia stem-end rot, sour rot, and green mold. Materials and Methods In vitro evaluation of the fungicides was performed by incorporating the materials into sterilized Difco potato de xtrose agar before pouring the media into petri dishes. After the media solidified, the plates were inoculated with the various fungi as described previously (10). Inhibition of growth was measured for each fungus at specific times after inoculation when substantial growth had occurred on the control plate. Proc. Fla. State Hort. Soc. 98: 1985. The materials for the tests described in this study were obtained from the following sources: DuPont and Co.—benomyl (Benlate, 50% w.p.); FMC Corporation—thiaben dazole (TBZ, 5% ex.), Janssen Pharmaceutica—imazalil (68% e.c); Ciba Geigy Co.—etaconazole (CGA 64251, 13.5% e.c); Kenogard—guazatine (Panoctine, 40% e.c); Maag Agrochemicals—fenpropimorph (R014-3169, 25% e.c); Sigma Chemical Co.—sorbic acid, potassium salt; Chemie Research and Manufacturing Co., Inc.—DF-100 (50% e.c); and Bio Vet Corporation—Purogene (2% aque ous stabilized chlorine dioxide). Available chlorine dioxide was prepared by reacting sodium chlorite with muriatic acid (5). Stem-end rot and green mold developed from natural infections. The incidence of green mold in these tests was high because the Valencia fruit suffered freeze injury and the spore population of Penicillium digitatum produced on damaged, dropped and infected fruit was quite high in the grove. Fruit were inoculated with Geotrichum candidum to induce sour rot. Spores of this organism were adjusted to a concentration of 10fi in 10 fig/ml of cycloheximide (6) and 5 fxl of the suspension were injected to a depth of 3 mm into the rind at one site on the equator of each fruit. Fungicide treatments were applied to oranges {Citrus sinensis (L.) Osb. cv. Hamlin and Valencia). The Hamlin oranges were degreened with 5-10 ppm ethylene at 8486°F with 92-94% relative humidity for 50 to 72 hours. Fruit were washed, graded, and randomized into the vari ous replicated lots. The fungicides were applied at specific rates in a nonrecovery spray to fruit rotating on horsehair brushes saturated with the fungicide. The fruit were dried at 120°F, waxed with a solvent wax, and packed in 2/5 bushel cartons. The Fruit were stored near 70°F at relative humidities of 85-90% for 1-3 weeks. Results and Discussion In vitro tests. Several of the fungicides were quite inhibit ory to growth of some pathogens at a low concentration of 1 ppm (Table 1). Benomyl, thiabendazole, and imazalil, which are presently registered for postharvest treatment of fruit, exhibited superior activity against Penicillium (green mold) and Diplodia and Phomopsis (stem-end rot). Benomyl and thiabendazole were also quite effective in-re ducing the growth of Colletotrichum, which causes anthracnose in some cultivars. The experimental materials fen propimorph, guazatine, and etaconazole effectively inhi bited growth of P. digitatum at a concentration of 1 ppm, but except for fenpropimorph, they were less effective against Diplodia and Phomopsis. Guazatine and etaconazole were the only materials that significantly inhibited growth of Geotrichum candidum (sour rot). Potassium sorbate, Purogene and DF-100 were much less inhibitory at 1 ppm than the other fungicides. How ever, use of higher concentrations of these less fungitoxic materials might improve their fungicidal effectiveness. Phytophthora citrophthora Sm. and Sm., the brown rot or ganism, was not inhibited by any of the fungicides applied at 1 ppm. The growth of Alternaria citri Ellis and Pierce (black rot) was inhibited best by fenpropimorph. Some of the materials showed no activity against this fungus. Control of Diplodia stem-end rot. Benomyl and thiaben dazole effectively and consistently controlled this organism in degreened oranges (Table 2). Significant control also was usually obtained with guazatine and imazalil, but the degree of control was not as good as that obtained with the benzimidazole fungicides. Effective control of Diplodia stem-end rot was not obtained with potassium sorbate, though it has controlled stem-end rot caused by Phomopsis in other studies (15). Low levels of DF-100 appeared to provide some control of stem-end rot in Test 1 but not in Test 3. The higher concentrations of DF-100 evaluated in Test 2, however, did not significantly reduce decay. There was some indication that propylene glycol, which com prises 25% of the DF-100 product, may possess some activ ity against Diplodia. In previous studies (4), the effectiveness of guazatine and benomyl for the control of Diplodia was improved when the materials were applied before, rather than after, degreening. Similar improvement could be expected with the other materials used in these tests. Under commercial conditions, predegreening treatments are most easily applied in a drench application. Control of sour rot. Sour rot was controlled with treat ments of guazatine and etaconazole (Table 3). These ex perimental fungicides have proven effective in other studies (1, 2, 7, 13, 14). Guazatine has been used in Au stralia for several years, and is much more effective for sour rot control than sodium o-phenylphenate (14). How ever, availability of guazatine to Florida packers for decay Table 1. Inhibition of different fungal pathogens on potato dextrose agar containing 1 jxg/rnl of certain fungicides. Percent inhibition radial growth Fungicide Penicillium digitatum' Penicillium ilalicum Phomopsis citri Alienlaria citri Diplodia natalensis Phytophthora citrophthora Geotrichum candidum Collelotrkhum gloeospo rio ides Benomyl Thiabendazole Fenpropimorph