EVALUATION OF NUTRITIONAL PRODUCTS AND FUNGICIDES FOR CONTROL OF CITRUS GREASY SPOT

Transcription

1 Proc. Fla. State Hort. Soc. 108: EVALUATION OF NUTRITIONAL PRODUCTS AND FUNGICIDES FOR CONTROL OF CITRUS GREASY SPOT L. W. TlMMER AND S. E. ZlTKO University of Florida, IFAS Citrus Research and Education Center 700 Experiment Station Road Lake Alfred, FL Additional index words. Mycosphaerella citri. Abstract In field tests on Ruby Red grapefruit, the fungicides copper hydroxide (Kocide DF), benomyl (Benlate 50 DF), citrus spray oil, and fenbuconazole significantly reduced greasy spot severity and defoliation. Iprodione (Rovral 4 SC), fosetyl-ai (Aliette 80 WP), fluazinam, and myclobutanil did not control the disease or suppressed it only slightly. Of the nutritional prod ucts, Keyplex 350 at 2.0 gal/acre or at 0.5 gal/acre with citrus spray oil provided good control of greasy spot in field tests. Keyplex 445, Goemar MZ 63, and Tracite N also controlled greasy spot in field tests. In greenhouse tests with artificially inoculated rough lemon seedlings, copper hydroxide reduced greasy spot severity and defoliation much more than Keyplex 350, whether products were applied before or after inoculation. Tracite Zinc, Iron, and Manganese each suppressed greasy spot on inoculated rough lemon seedlings, suggesting the metals contained in nutritional products may be the active agents in these materials that suppress greasy spot develop ment. The standard recommendation for control of greasy spot caused by Mycosphaerella citri Whiteside is the application of copper fungicides and/or citrus spray oil in the summer (Kriapp, 1995). Copper fungicides are highly effective for re ducing greasy spot severity and disease-induced defoliation (Whiteside, 1984). However, copper can accumulate to phytotoxic levels in citrus soils so the ph must be maintained at or above to avoid toxicity problems (Alva and Graham, 1991). Application of copper fungicides, particularly in sum mer, may darken or enhance existing blemishes such as windscar, melanose, or injuries to the fruit rind (Brodrick, 1970). Since copper fungicides are also used to control melanose, scab, and Alternaria brown spot (Knapp, 1995), the total amount of copper applied to citrus groves annually may reach high levels. Citrus spray oil may be used as a substitute for copper fun gicides but is not as effective, especially for more susceptible species such as grapefruit (Whiteside, 1984). In the past, benomyl was commonly used as an alternative to copper fun gicides in the summer spray, but M. citri eventually developed resistance to this fungicide (Whiteside, 1980). Captafol was also effective for greasy spot control but was not compatible with oil, and its use is now prohibited. An earlier study (Timmer and Zitko, 1994) indicated that certain nutritional prod ucts were effective in reducing greasy spot severity. The purpose of the present study was to further investigate the usefulness of nutritional products and to evaluate new fungi- Florida Agricultural Experiment Station Journal Series No. N Sup ported in part by the Florida Citrus Research Advisory Council Project No Proc. Fla. State Hort. Soc. 108: cides as potential replacements for copper fungicides in the summer applications for greasy spot control. Materials and Methods Field experiments. All field experiments were conducted in 3 to 8-yr-old groves of Ruby Red or Marsh grapefruit (Citrus paradisi Macf.) trees near Lake Alfred, Florida. The Ruby Red grove was planted on a 15 x 25-ft spacing and the Marsh grove on a 15 x 20-ft spacing. Applications of fungicides and nutri tional products were made with a handgun sprayer at psi using about 1-2 gal per tree depending on tree size. All products were applied to the tree canopy except ByPass, which was applied to the soil surface under the canopy. Rates in Tables 1-4 are presented as the equivalent amounts needed to treat an acre of mature grove. All treatments were arranged in a randomized complete block design. In Experiments 1 and 2 (Tables 1 and 2), ten single-tree replications were used for each treatment. In Experiments 3 and 4 (Tables 3 and 4), five replications of two-tree plots were used. In April of each year, 10 spring flush shoots with about 8-12 leaves each were tagged on each tree in the experiment for subsequent evaluation of greasy spot severity and defoliation. The following winter, greasy spot severity was rated on each tagged shoot on a scale of 0 = none; 1 = negligible to 5%; 2 = 6-10%; 3 = 11-15%; 4 = 16-20%; and 5 = >20% of the leaf area covered by actual greasy spot lesions. Ratings were based on comparisons with photographs where the actual percentages of the leaf area affected had been determined by computerassisted image analysis. The chlorotic area surrounding the le sions was not considered in the rating. The percentage of the total number of leaves which had fallen was recorded for each shoot and expressed as percentage defoliation. The overall density of the canopy was rated on a scale of 1-10, where 1 = very thin canopy, little old foliage remaining, to 10 = dense canopy, minimal leaf drop. Dates of evaluations are given in Tables 1-4. Screenhouse experiments. For the three screenhouse experi ments, rough lemon (C. jambhiri Lush.) seedlings were grown in the greenhouse in 1-gal plastic pots as a single stem about 3 ft tall. Seedlings were then topped leaving mature, fully-expanded leaves for inoculation. All new growth was re moved from the plants during the course of the experiments. For artificial inoculation, inoculum was produced follow ing the method of Whiteside (1974). Briefly, cultures of M. cit ri were grown in modified liquid Fries medium for 1 to 2 months. The mycelium was collected on cheesecloth, washed several times with distilled water, and fragmented in a blend er. Sucrose was added to the inoculum preparation to a final concentration of 1.0%. The inoculum suspension was applied to the underside of leaves using a pressurized hand sprayer. Inoculated plants were allowed to dry for 3 to 4 hr and then were placed in a mist chamber and leaves maintained moist for 48 hr. Subsequently, plants were transferred to the screenhouse where they were exposed to ambient conditions for the remainder of the experiment. Nutritional products and fungicides were sprayed to run off using a hand-pump applicator. In some cases, applications were made 10 days prior to inoculation but, unless otherwise 83

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5 effective to be recommended for greasy spot control. Of the conventional fungicides, fenbuconazole is the most promis ing as a replacement for copper fungicides for greasy spot control. It is highly effective at very low rates, especially when used in combination with spray oil. Nutritional products were surprisingly effective for con trol of greasy spot, at least in some tests. Nearly all of the prod ucts showed at least some suppression of greasy spot. Nutritional products were only slightly less effective than cop per fungicides in the field experiments, but were much less effective than copper in screenhouse experiments where the inoculum levels were high and conditions were favorable for disease development. Activity of non-conventional materials against Mycosphaerella citri is not without precedent. Timmer etal. (1980) and Whiteside (1983) demonstrated that many acaricides have some activity with dicofol, chlorobenzilate and cyhexatin being the most effective. The mechanism by which these materials act is open to question. These products could stimulate the natural defens es of the host, inducing production of anti-fungal compounds by the plant (Niedz, et al., 1994). However, these products may be directly toxic to the fungus. Most active materials, in cluding the acaricides, contain heavy metals which could act directly on the fungus. Keyplex 350 was effective when ap plied both before and after inoculation. The same is true of most conventional fungicides, which when applied before in fection inhibit ascospore germination, or if applied later kill the extramatricular hyphae on the underside of the leaf (Wh iteside, 1971). Thus, we suspect that any of the nutritional products which contain sufficient minerals would suppress greasy spot. In tests of single metals, zinc seemed to be the most effective. Additional research will be needed to deter mine the mechanism of action. Although not as effective as copper fungicides, many of these products could be substituted where growers wish to avoid applying more copper to their groves. Relatively high rates are needed for effective control. Keyplex 350, the prod uct we tested most extensively, was most effective at gal/acre and only marginally effective at 0.5 gal/acre. The cost of 1.0 gal of Keyplex 350 is comparable to the cost of cop per fungicide applied at the recommended rate of 4 lb metal lic copper per acre. Given the apparent increase in greasy spot severity and defoliation when nitrogen was added to Key plex 350, use of additives such as urea and potassium nitrate should be avoided. Tracite Zinc and Keyplex 445, though less extensively tested, also appeared promising. Used in combi nation with 5 gal/acre of spray oil, these products should pro vide satisfactory control in most situations. In these tests, there was insufficient greasy spot rind blemish to evaluate damage to fruit. Thus, it will probably still be necessary to ap ply summer sprays of copper fungicides for rind blotch con trol where it is a problem. Literature Cited Alva, A. K. andj. H. Graham The role of copper in citriculture. Adv. Agron. 1: Brodrick, H. T Accentuation of blemish marks by copper fungicide sprays. S. Afr. Citrus J. 441:27-29, 31. Knapp, J. L., ed Florida Citrus Pest Management Guide. Univ. Florida, Inst. Food & Agric. Sci., Coop. Ext. Serv. Publ. Sp pp. Niedz, R. P., H. Doostdar, T. G. McCollum, R. E. McDonald, and R. T. Mayer Plant defensive proteins and disease resistance in citrus. Proc. Fla. State Hort. Soc. 107: Timmer, L. W., R. J. Reeve, and R. M. Davis Epidemiology and control of citrus greasy spots on grapefruit in Texas. Phytopathology 70: Timmer, L. W. and S. E. Zitko Evaluation of fungicides for control of citrus greasy spot. Fungicide and Nematicide Tests 49:377. Whiteside, J. O Effectiveness of spray materials against greasy spot in relation to time of application and infection periods. Proc. Fla. State Hort. Soc. 84: Whiteside, J. O Environmental factors affecting infection of citrus leaves by Mycosphaerella citri. Phytopathology 64: Whiteside, J. O Tolerance of Mycosphaerella citri to benomyl in Florida citrus groves. Plant Dis. 64: Whiteside, J. O Fungicide effects of some acaricides on Mycosphaerella citri Plant Dis. 67: Whiteside, J. O Reliability of spray treatments for reducing greasy-spot induced defoliation on grapefruit trees. Proc. Fla. State Hort. Soc. 97: Proc. Fla. State Hort. Soc. 108: