African Journals Online
African Plant Protection

Volume 7 Number 2, 2001
Abstracts

Identification of three viruses from Pisum sativum in South Africa
Jooste, A.E.C.Pietersen, G.Kasdorf, G.G.F.Goszczynski, D.E.

Abstract: Viruses were isolated from three pea (Pisum sativum) plants from different regions in South Africa. Plant 91/0394, from George, Western Cape Province, displayed inter-veinal chlorosis of the upper leaves with chlorotic flecks and downward leafcurl. Virus isolated from this plant was identified as Pea enation mosaic virus (PEMV) based on ELISA tests. The second plant, 94/1969, from Brits, North West Province, showed severe mosaic symptoms. Broad bean wilt virus (BBWV) was identified from it by immuno-electron microscopy. The third plant, 95/0931, from Pretoria, Gauteng Province, displayed yellow mosaic and vein-clearing symptoms and flexuous potyvirus-like particles were seen when viewed with an electron microscope. The nucleotide sequence of the 3' untranslated region (3' UTR) and part of the coat protein gene of the virus isolated was 98.5 % homologous to the Pea mosaic virus-I(PMV-I) strain of Bean yellow mosaic virus(BYMV). The first two viruses (91/0394 and 94/1969) represent viruses not previously reported on pea in South Afica and the other isolate (95/0931) confirms previous reports of BYMV on pea in South Africa.

Monitoring and control of coffee bean weevil, Araecerus coffeae (Coleoptera : Anthribidae), in southern African citrus orchardsGrout, T.G.Begemann, G.J.Stephen, P.R.

Abstract: Araecerus coffeae has recently been recovered from citrus fruit in low altitude areas of Swaziland and Mpumalanga province, South Africa. Although this pest is cosmopolitan and therefore not a phytosanitary problem, infested fruit may still be rejected for export due to the presence of live insects. An investigation of alternative host plants showed that the beetles completed their life cycle in fruit from mango (Mangifera indica), marula (Sclerocarya birrea), Natal mahogany (Trichilia emetica), Cape ash (Ekebergia capensis), forest toad tree (Tabernaemontana ventricosa) and in old sugar cane (Saccharum officinarum) stems. The beetles were monitored in citrus orchards using sticky pipe traps containing either a cut orange or caged male A. coffeae. Caged virgin female beetles on yellow sticky traps were not attractive. Contact mortality from various insecticides was assayed by dipping adult beetles in diluted insecticide for 30 seconds. The most effective products tested were cypermethrin, deltamethrin, dichlorvos, profenofos and trichlorfon. Other bioassays were conducted in large cages using baits comprising chopped orange and either methiocarb, tartar emetic, trichlorfon or pyriproxyfen as insecticide. Laboratory and orchard evaluations with the insect growth regulators pyriproxyfen, teflubenzuron and triflumuron gave unsatisfactory results. Trials where chopped oranges on the orchard floor were sprayed with tartar emetic were evaluated using traps. This may be the most suitable treatment for IPM orchards, although it did not provide complete control during the evaluation period.

Horizontal distribution of root nematode damage in six Musa spp. genotypes
Blomme, G.De Beule, H.Swennen, R.L.Tenkouano, A.De Waele

Abstract: Root necrosis was assessed at different distances from the corm for six different Musa genotypes and the effect of planting material as well as plant developmental stage on the occurrence of root necrosis, investigated. Radopholus similis and Helicotylenchus spp. were of predominant importance in root destruction as their population densities were significantly related to the root necrosis index. A gradual decrease in percentage root necrosis with increasing distance from the corm was observed for both mother plants and tallest suckers. Hardly any root necrosis was observed beyond 60 cm from the corm for roots of both mother plants and tallest suckers. Severity of root necrosis was influenced by genotype and type of planting material. `Yangambi km5' did not develop excessive root necrosis. However, relatively high overall nematode population densities were evident in sword sucker-derived `Yangambi km5', which may indicate tolerance to nematode infection. By contrast plantains had a high percentage root necrosis despite moderate nematode population densities, indicating a high sensitivity of these genotypes. Similar root necrosis percentages were observed for both in vitro and sucker-derived plants of `Yangambi km5', although nematode numbers were considerably higher for the sucker-derived plants. This may indicate a higher sensitivity to nematodes in the in vitro-derived plants compared to the sucker-derived plants.

Plant-parasitic nematodes associated with maize–bean intercropping systems in Kenya
Van den Berg, E.Marais, M.Kandji, S.Mounport, D.N'Diaye, B.Cadet, P.

Abstract: Identifications of plant-parasitic nematodes from maize–bean intercropping systems in the western highlands of Kenya revealed the presence of 17 species belonging to ten genera, viz. Ditylenchus sp., Helicotylenchus dihystera, H. pseudorobustus, Hemicriconemoides snoecki, Meloidogyne incognita, M. javanica, Paratrichodorus minor, Pratylenchus brachyurus, P. zeae, Rotylenchulus borealis, Scutellonema brachyurus, S. clathricaudatum, S. magniphasmum, Tylenchorhynchussp., Xiphinema elongatum, X. pinoides and X. set ariae. The species are discussed in relation to their occurrence in some neighbouring countries.

Reinfestation by nematodes and performance of hot-water-treated East African Highland cooking bananas as perceived by farmers in Ikulwe, Iganga district, Uganda
Speijer, P.R.Nampala, P.M.Elsen, A.Ekwamu, E.De Waele, D.

Abstract: On-farm trials were carried out on nine farms at Ikulwe, Iganga district, Uganda, to evaluate reinfestation by nematodes and plant growth performance of `clean' (pared and hot-water-treated) cooking banana planting material under local production conditions. Radopholus similis and Helicotylenchus multicinctus were the most abundant nematode species on the farms. Also present in lower numbers were Meloidogyne spp. and Pratylenchus goodeyi. Thirty months after planting, nematode numbers in the treated plants were still lower than in the untreated plants. The root systems of pared and hot-water-treated plants were healthier than those of untreated plants as indicated by the lower percentage dead roots and root necrosis of the functional primary roots. Over a period of 30 months, 12 % of the treated plants had died compared to 17 % of the untreated plants. Over 50 % of the farmers interviewed considered that clean planting material resulted in plants that flowered earlier, produced more suckers, had a longer life span, showed increased vigour and bunch size, were less prone to toppling and had a better food quality.

A review of early blight of potato
Van der Waals, J.E.Korsten, L.Aveling, T.A.S.

Abstract: Early blight of potatoes, causal agent Alternaria solani, causes major yield losses in most potato growing areas of the world. Leaf symptoms are characteristic dark brown to black lesions with concentric rings. In this review morphological, physiological and molecular characters of the pathogen as well as the disease cycle, epidemiology and control of the disease are discussed. The early blight situation on potatoes in South Africa is also summarised.

Effect of packhouse procedures on the viability of Phyllosticta citricarpa, anamorph of the citrus black spot pathogen
Korf, H.J.G.Schutte, G.C.Kotzé, J.M.

Abstract: Conidia of Phyllosticta citricarpa (Pc) did not infect unwounded packhouse-treated citrus fruit maintained at –0.5 and 25 ºC in artificial and natural inoculation studies, but infrequently produced black spot lesions from which the pathogen could be isolated on artificially inoculated wounded fruit kept at 25 ºC. Conidial germination and appressorium formation by seven Pc isolates from various countries were completely inhibited in vitro by the postharvest fungicides guazatine and o-phenylphenol (sodium salt) at recommended rates, whereas thiabendazole, imazalil sulphate, two emulsion formulations of imazalil, and particularly prochloraz, provided significant inhibition. Chlorine dioxide suppressed conidial germination more effectively and at lower concentrations than calcium hypochlorite. Packhouse treatments which consistently reduced the viability of Pc in black spot lesions on fruit included warm water (43–47 ºC for three minutes), chemical tank (guazatine, imazalil sulphate, 2,4-D), and a combination of chlorine, high-pressure spraying, warm water, chemical tank, and polyethylene waxing. Conidial viability was reduced to zero on black spot-infected fruit stored for three weeks at 25 ºC, as well as on fruit exposed to chlorine, warm water, the above chemicals, or all treatments combined.

Pythium / Rhizoctonia complex causing damping-off of cowpea in South Africa
Adandonon, A.Aveling, T.A.S.Labuschagne, N.Ahohuendo, B.C.

Abstract: Pythium ultimum and Rhizoctonia solani were consistently isolated from cowpea seedlings with symptoms of damping-off in South Africa. Isolates of the two species were tested individually and in combination for their effect on cowpea seedlings in the greenhouse at 20, 26 and 32 ºC. Damping-off induced by R. solani was the highest at 20 ºC, and disease incidence decreased with increase in temperature. P. ultimum caused most disease at 26 ºC and significantly less at 20 and 32 ºC. Percentage damping-off induced by P. ultimum was higher than that induced by R. solaniat all three temperatures. Disease incidence incited by P. ultimum was significantly reduced by R. solani when the two fungi were combined, suggesting an antagonistic interaction between them.

First report of Mycosphaerella areola, teleomorph of Ramulariopsis gossypii, on cotton in South Africa
Gouws, M.A.Prinsloo, G.C.Van der Linde, E.J.

Abstract: Ramulariopsis gossypii, which causes false mildew in cotton, was isolated in South Africa. Fruiting-bodies resembling those of the teleomorph, Mycosphaerella areola, were observed on decaying cotton leaves and bolls. Artificial inoculation studies confirmed the relationship between the anamorph and the newly-discovered teleomorph.