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EVALUATION OF DIFFERENT SEED TREATMENT/SOIL DRENCH COMBINATIONS FOR THE CONTROL OF DAMPING-OFF OF ROOIBOS TEA ( ASPALATHUS LINEARIS) IN NURSERIES

SC LAMPRECHT1 and S DENMAN2

1ARC-Plant Protection Research Institute, Stellenbosch, and 2Department of Plant Pathology, University of Stellenbosch, Stellenbosch, South Africa

Background and objectives
Damping-off of rooibos tea [Aspalathus linearis (Burm. f.) R. Dahig.] seedlings causes serious losses in nurseries in the Western Cape Province in South Africa. In field trials mean losses of 53% were recorded in plots where no fungicide treatments were applied. Pathogenicity tests showed that of the 23 fungal species tested, 18 caused damping-off of seedlings in artificially infested soil. Thus damping-off of rooibos tea seedlings is caused by a complex of fungi of which Fusarium oxysporum, F. solani, Pythium irregulans and Rhizoctonia solani are the most important. The thiram seed treatment and captab soil drench used by farmers increased the survival of seedlings compared with untreated seed and soil, but serious losses are still recorded. Thiram seed treatment is relatively effective against damping-off caused by Fusarium spp. [1], but not by Pythium spp. [2]. Preliminary evaluations under growth-room conditions on the effect of seed treatments, soil drenches and application times of fungicide soil drenches showed that treatments that contained metalaxyl as one of the components were more effective in reducing damping-off than treatments without metalaxyl. The aim of this study was to evaluate selected seed treatments, soil drenches and application times for soil drenches under field conditions for the control of damping-off of rooibos tea seedlings in nurseries.

Materials and methods
20 seed treatment/soil drench combinations were evaluated for the control of damping-off of rooibos tea seedlings at a nursery in the low (180-250 mm/year) and in the high (>250-400 mm/year) rainfall areas in the Clanwilliam district in the Western Cape Province. Metalaxyl, thiram and toclofos methyl were used in different combinations for seed treatment, and benomyl, captab, metalaxyl and toclofos methyl in different combinations for soil drenches. Application times for soil drenches that were evaluated were: the day before planting, and 3, 7, 10 and 24 days after planting. The survival, plant length, dry mass, foliage and root disease severity and incidence of pathogenic fungi on different plant parts of seedlings were determined 3 months after planting.

Results and conclusions
The survival rate of seedlings in the low-rainfall area was significantly (P=0.05) higher than in the high-rainfall area. Three of the treatment combinations (13, 15, 19) resulted in significantly better survival rates compared with the control (1=untreated seed and soil). The mean survival of treatments 1 (control), 13, 15 and 19 were 23, 62, 63 and 64%, respectively. Plant length, dry mass and disease severity of seedlings subjected to treatments 13, 15 and 19 did not differ significantly from treatment 1. This confirms that the fungicides used for seed and soil treatment in treatments 13, 15 and 19 were not phytotoxic. Treatment 19 proved to be the most economic and resulted in significant economic benefits. The incidences of the most important damping-off pathogens, viz. F. oxysporum, F. solani, P. irregulans and R. solani were reduced in roots of seedlings subjected to treatment 19 compared with control seedlings. This study showed that seed treatments and soil drenches are important components of an integrated control strategy against damping-off of rooibos tea seedlings in nurseries. The treatment of seed and soil with multi-component fungicides significantly increased survival, reduced the incidence of important pathogenic fungi in roots, was not phytotoxic and had measurable economic benefits.

References
1. Lamprecht SC, Marasas WFO, Knox-Davies PS, Calitz FJ, 1990. Phytophylactica 22, 201-208.
2. Denman S, 1992. MSc thesis, University of Stellenbosch, South Africa.