Reduction of contact fungicide tank rates by addition of a patented DDAC (Sporekill+) for the control of Guignardia citricarpa and Alternaria alternata on citrus in South Africa

• Published in: Journal of plant pathology Vol. 90; no. 3; p. 601
• Main Author: Schutte, G C
• Format: Journal Article
• Language: English
• Published: 01.11.2008
• Subjects: Alternaria; Alternaria alternata; Citrus; Guignardia
• ISSN: 1125-4653

Control of citrus foliar diseases such as citrus black spot (CBS) and Alternaria brown spot (ABS) caused by Guignardia citricarpa Kiely and Alternaria alternata pv. citri (Fr:Fr) Keissl. respectively, are entirely dependent on the application of fungicidal sprays in South Africa during susceptible growth period. The critical period for CBS infection is from fruit set until late summer (October to January for southern hemisphere) and is limited to orange, grapefruit and lemon cultivars. On the other hand, the critical period for ABS is from the first spring flush until harvest (September to April) and infects only mandarins and its hybrids. Copper compounds and mancozeb are some of fungicides registered in South Africa for control of both diseases. However, frequent spraying of copper fungicides can result in fruit stippling, accentuation of blemishes and accumulation in soils. Ethylenebisdithiocarbamate fungicides such as mancozeb have been associated with possible human health hazards. There is an urgent need for new chemicals to control these diseases or to reduce the rates of the existing registered fungicides through tank mixing with less toxic fungicides. Two Valencia orchards with a high incidence of CBS were selected at Croc Valley Citrus Co. and Friedenheim Estates east of Nelspruit, South Africa. A randomized block design with 5 single-tree plots per treatment was used. Four successive fungicides were applied with a trailer-mounted, high-volume, high-pressure (2,500 - 3,000 kPa) sprayer with two hand-held spray guns. Spray volumes varied according to the size and canopy density of the tree but all trees were sprayed to the point of run-off. Currently, all commercial fungicide applications for CBS control in South Africa begin in mid-October, based on research findings from ascospore releases and spore trap data. At fruit maturity in July or August, CBS severity was rated on 100 fruits per tree according to a 3-point index: 0 = clean fruit with no CBS lesions; 1 = one to three CBS lesions per fruit; and 2 = four or more CBS lesions per fruit. Proportional data were analysed by ANOVA, using Fisher's LSD test (P = 0.05). One 'Nova' mandarin trial site with a high incidence of ABS was selected at Belmont Estates west of Nelspruit, South Africa. The same method of application and the same fungicides were used as described above. A total of 7 to 8 applications were made at monthly intervals between September and March/April. The evaluation of brown spot (200 fruit per replicate) on the fruit rind was conducted just prior to harvesting, during mid-April. Criteria used for rating the fruit were: 0 = fruit with no brown spot lesions, 1 = fruit with one to five brown spot lesions, 2 = fruit with six or more brown spot lesions. Fungicides that were tested were: Dimildex, 850 g/l WP (copper oxychloride, Delta Chemicals, L 5094), Du Pont Kocide 2000, 538 g/kg WG (copper hydroxide, Du Pont), Dithane M45, 800 g/kg WP (man-cozeb, Dow AgroSciences, L 2914), Copper Count N (copper ammonium acetate, 316 g/l SL, Hygrotech, L 2602) and Sporekill (didecyl dimethyl ammonium chloride 120 g/l SC, ICA International Chemicals, L 7115). Mancozeb, copper oxychloride and copper hydroxide treatments at registered rates and halved rates of the latter fungicides in tank mixtures with Sporekill (100 ml/hl water), were equally effective for the control of CBS. Sporekill on its own tested at rates of 50, 100 and 200 ml/hi water did not give the desired control and can therefore not be recommended for CBS control on its own. For Alternaria brown spot control, where the copper oxychloride, copper hydroxide and mancozeb fungicide rates were lowered from 200 g/hl water to 100 g/hl water combined in tank mixtures with Sporekill (100 ml/hl water) and sprayed at monthly intervals (7 applications), the results were just as good. Both the copper fungicide treatments with Sporekill resulted in 98% clean exportable fruit. No stippling by to copper was observed in the latter treatments with Sporekill. Reducing the rates of copper fungicides and mancozeb with 50% can be achieved if it is combined with a sanitising agent such as Sporekill for the control of foliar citrus diseases such as CBS and ABS. These combinations will be helpful especially in those countries where there is a drive to limit the annual usage of copper to 8 kg/ha. These lowered rates of copper and mancozeb in tank mixtures with Sporekill are registered for the control of both citrus black spot and Altemaria brown spot in South Africa.