T.A.S. Aveling

Influence of environmental factors on field concentrations ofAlternaria solani conidia above a South African potato crop

Trends in weather variables and concentrations of airborne conidia ofAlternaria solani were monitored in a potato field in South Africa during three potato-growing seasons in 2001 and 2002. Distinct seasonal variation was noted, with a drop in spore numbers during winter. Peaks in spore concentration coincided with periods favorable for spore formation and dispersal; most notable was the effect of interrupted wetting periods. Diurnal periodicity of spore dispersal was also observed, with the peak of spore concentrations between 9h00 and 18h00. Few spores were sampled at night, when wind velocity and temperature are lowest and relative humidity is highest. Increased numbers of spores were sampled during days of harvesting or when other ground-operated farm equipment was used. The results obtained in this study will be useful in establishing decision support systems to control early blight on potatoes in southern Africa.

Infection process of Colletotrichum dematium on cowpea stems

Colletotrichum dematium is the cause of a new fungal stem disease on cowpea in South Africa. The pre-penetration and infection process of this fungus on cowpea stems was studied by light microscopy and SEM. Conidia began germinating at 6 h post inoculation (hpi), forming appressoria directly or at the ends of germ-tubes. By 14 hpi appressoria had melanized and direct penetration of host tissue had begun. At 20 hpi infection vesicles formed in epidermal cells. Thick, knotted primary hyphae formed from these vesicles and entered adjacent cells. At ‡40 hpi C. dematium produced secondary hyphae which were highly branched, and grew extensively inter- and intracellularly. After approximately 48 hpi light brown lesions appeared on the stem, associated with the invasion of secondary hyphae into cells and cell necrosis. Acervuli with one or two melanized setae were visible on lesions by 70 hpi.

Genetic variation among Sclerotium isolates from Benin and South Africa, determined using mycelial compatibility and ITS rDNA sequence data

Damping-off and stem rot of cowpea caused bySclerotium rolfsii has previously been reported in Benin, where the pathogen showed variation in growth and sclerotia production among isolates. Pathogenicity, mycelial compatibility group (MCG) tests and rDNA sequence analyses were conducted on different isolates ofS. rolfsii and S. delphinii collected from different hosts and geographical areas in Benin and South Africa. All the isolates, when inoculated into soil and planted with cowpea, caused damping-off and stem rot symptoms. Aggressiveness among isolates varied depending on the host from which each was isolated. Isolates originating from cowpea produced the highest disease incidence followed by isolates from peanuts. Four MCGs were distinguished among 66 isolates. Isolates from the same hosts tended to group into the same MCG. The incidence of damping-off and stem rot of cowpea, expressed as percentage diseased plants, varied among MCGs. Plants inoculated with MCG2 displayed the highest disease incidence, whereas MCG4 resulted in the least. Parsimony analysis of ITS DNA sequence data supported a close affinity of the Sclerotium spp. but showed genetic variation among isolates with no grouping based on host of origin.

Infection studies of Stemphylium vesicarium on onion leaves

Onion leaves were inoculated with conidia of Stemphylium vesicarium . Conidial germination, appressorial formation and penetration of the leaf surface were studied. Several germ-tubes developed from each conidium and grew in any direction across the leaf surface. Terminal (29·7%) or intercalary (18·8%) appressoria were formed above epidermal cells. Of these appressoria, 72·3 and 47·7%, respectively, successfully penetrated epidermal cells. Single, terminal (35·9%) and double (12·5%) appressoria formed above stomata and 96·5 and 89·4% of these appressoria, respectively, successfully penetrated the leaf via stomata. Occasionally compound appressoria were formed above stomata (3·1%) with 100% successful penetration. Penetration of the leaf surface, whether directly through the epidermis or via stomata, only occurred after the formation of appressoria.

Phytotoxic effects of fumonisin B1 on cowpea seed

The cultivation of cowpea (Vigna unguiculata) plays a vital role in the livelihood of many subsistence farmers and rural communities in tropical and subtropical countries. The seeds are prone to fungal infestation and mycotoxin contamination during sub-optimal storage conditions. Fumonisin B1 (FB1), produced byFusarium proliferatum, has been detected in cowpea seeds. Surface-disinfected seeds were imbibed in sterile distilled water amended with FB1 at various concentrations. Percentage germination was determined according to the International Seed Testing Association rules. All the toxin concentrations significantly decreased seed germination and the two highest concentrations — 50 and 100µg ml−1 FB1 — inhibited root and shoot elongation. FB1-treated embryonic tissues evinced compaction of the protoplasm and separation of the plasmalemma from the cell wall. Lipid bodies accumulated, which seemed to be lining the cell wall. This is the first study to demonstrate the phytotoxic effects of FB1 on cowpea seeds.

Infection studies of Alternaria cassiae on cowpea

Alternaria cassiae is the causal pathogen of a new, destructive foliar disease of cowpea (Vigna unguiculata). Cowpea leaves were inoculated with conidia of A. cassiae. Conidium germination, appressorium formation, penetration and colonisation of the plant surface were studied using light and scanning electron microscopy. Multiple germ-tubes developed randomly from each conidium and grew in any direction across the leaf surface. Terminal (71%) or intercalary (29%) appressoria were formed above epidermal cells or over stomata. Penetration of the plant surface, whether directly through the epidermis or via stomata, occurred with or without the formation of appressoria. Following penetration through a stoma, bulbous primary hyphae developed within the sub-stomatal cavity. Secondary hyphae developed from the primary hyphae and grew within the intercellular spaces, penetrating epidermal and mesophyll cells. Conidiophores emerged directly through the epidermis or through stomata.

Screening of plant extracts for antifungal activities against Colletotrichum species of common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata (L.) Walp)

SUMMARY The aim of the present investigation was to evaluate the antifungal activities of plant extracts which can be used to control bean and cowpea anthracnose. Acetone, ethyl acetate and water extracts of Ipomoea batatas, Carica papaya, Allium sativum, Syzygium cordatum, Chlorophytum comosum and Agapanthus caulescens were screened in vitro for their antifungal activities against Colletotrichum lindemuthianum and Colletotrichum dematium of common bean and cowpea using the agar disc infusion and microtitre double-dilution techniques. The same extracts were then tested for antifungal activity in vivo as seed treatments against anthracnose disease. The water extracts of Carica and Syzygium were active against C. lindemuthianum and had minimum inhibitory concentrations (MICs) of 1·56 mg/ml. Syzygium, Allium and Chlorophytum water extracts were active against C. dematium and MICs were 3·13, 6·25 and 12·5 mg/ml, respectively. The MICs of Allium, Syzygium and Agapanthus acetone extracts were 0·78, 3·13 and 6·25 mg/ml, respectively, against C. lindemuthianum and 0·78, 6·25 and 3·13 mg/ml against C. dematium. Agapanthus water extracts and all the acetone extracts tested in vivo effectively reduced the incidence and severity of bean anthracnose disease in the greenhouse. Agapanthus acetone, Allium water, and both acetone and water extracts of Carica and Syzygium performed well in vivo in reducing cowpea anthracnose disease and compared well with reductions due to the application of the synthetic fungicide fludioxonil+mefenoxam (the commercial product Celest ® XL) applied at 25 gai/l and also with levels in the non-inoculated control. The Agapanthus, Carica, Syzygium and Allium extracts were active on both Colletotrichum spp. in vitro and also reduced anthracnose disease of bean and cowpea and are potential seed treatments in anthracnose disease control. The easy seed treatment process and the accessibility of plants used in the present study could lead to high adoption of the use of the plant extracts as seed treatments by resource-poor, smallholder farmers.

Occurrence and distribution of cowpea damping-off and stem rot and associated fungi in Benin

SUMMARY Damping-off and stem rot of cowpea is an important soil-borne disease worldwide. Cowpea fields were randomly chosen in each agro-ecological zone in Benin and surveyed in 2001 and 2002 to determine the occurrence of the diseases throughout the country. Diseased plants, prevailing environmental conditions and cowpea grower cultural practices were recorded and causal agents associated with the disease identified. Results indicated that damping-off and stem rot were distributed throughout Benin. The disease incidence was higher in the South (0.074) and Centre zones (0.063) than in the other zones (<0.02) in the country. Among factors influencing the disease incidence, cultural practices such as sole crop and no-till systems appeared to be most important. Isolated fungi included Sclerotium rolfsii, Fusarium spp., Pythium ultimum, Rhizoctonia solani, Phoma sp., Rhizopus sp. and Trichoderma harzianum. None of the Fusarium, T. harzianum or Rhizopus sp. isolates were pathogenic in the greenhouse. Pythium ultimum, R. solani and Phoma sp. were infrequently isolated and few isolates caused the disease symptoms in the greenhouse. However, this is the first report of Phoma sp. causing damping-off and stem rot of cowpea in Benin. Sclerotium rolfsii was by far the most common species isolated from all the agro-ecological zones and all isolates were pathogenic on cowpea in the greenhouse. Sclerotium rolfsii was considered to be the main causal agent of cowpea damping-off and stem rot in the Republic of Benin due to its wide distribution, high incidence and predominance on plants with damping-off and stem rot symptoms.

Two Novel DNAs that enhance symptoms and overcome CMD2 resistance to cassava mosaic disease

ABSTRACT Cassava mosaic begomoviruses (CMBs) cause cassava mosaic disease (CMD) across Africa and the Indian subcontinent. Like all members of the geminivirus family, CMBs have small, circular single-stranded DNA genomes. We report here the discovery of two novel DNA sequences, designated SEGS-1 and SEGS-2 (for sequences enhancing geminivirus symptoms), that enhance symptoms and break resistance to CMD. The SEGS are characterized by GC-rich regions and the absence of long open reading frames. Both SEGS enhanced CMD symptoms in cassava (Manihot esculenta Crantz) when coinoculated with African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), or East African cassava mosaic virus-Uganda (EACMV-UG). SEGS-1 also overcame resistance of a cassava landrace carrying the CMD2 resistance locus when coinoculated with EACMV-UG. Episomal forms of both SEGS were detected in CMB-infected cassava but not in healthy cassava. SEGS-2 episomes were also found in virions and whiteflies. SEGS-1 has no homology to geminiviruses or their associated satellites, but the cassava genome contains a sequence that is 99% identical to full-length SEGS-1. The cassava genome also includes three sequences with 84 to 89% identity to SEGS-2 that together encompass all of SEGS-2 except for a 52-bp region, which includes the episomal junction and a 26-bp sequence related to alphasatellite replication origins. These results suggest that SEGS-1 is derived from the cassava genome and facilitates CMB infection as an integrated copy and/or an episome, while SEGS-2 was originally from the cassava genome but now is encapsidated into virions and transmitted as an episome by whiteflies. IMPORTANCE Cassava is a major crop in the developing world, with its production in Africa being second only to maize. CMD is one of the most important diseases of cassava and a serious constraint to production across Africa. CMD2 is a major CMD resistance locus that has been deployed in many cassava cultivars through large-scale breeding programs. In recent years, severe, atypical CMD symptoms have been observed occasionally on resistant cultivars, some of which carry the CMD2 locus, in African fields. In this report, we identified and characterized two DNA sequences, SEGS-1 and SEGS-2, which produce similar symptoms when coinoculated with cassava mosaic begomoviruses onto a susceptible cultivar or a CMD2-resistant landrace. The ability of SEGS-1 to overcome CMD2 resistance and the transmission of SEGS-2 by whiteflies has major implications for the long-term durability of CMD2 resistance and underscore the need for alternative sources of resistance in cassava.

Callose and β-1,3-glucanase inhibit Phytophthora cinnamomi in a resistant avocado rootstock

Phytophthora root rot (PRR) of avocado, caused by Phytophthora cinnamomi, is a significant threat to sustainable production wherever the crop is grown. Resistant rootstocks in combination with phosphite applications are the most effective options for managing this disease. Recently, the mechanisms underpinning PRR resistance have been investigated by the avocado community. Here, biochemical assays and confocal and scanning electron microscopy were used to investigate early defence responses in PRR resistant and -susceptible avocado rootstocks. Zoospore germination and subsequent hyphal growth for the pathogen were significantly inhibited on the surface of resistant avocado roots. When penetration occurred in the resistant R0.06 rootstock, callose was deposited in the epidermal cells, parenchyma and cortex of roots. In addition, b-1,3-glucanase was released early (6 h post-inoculation, hpi) in response to the pathogen, followed by a significant increase in catalase by 24 hpi. In contrast, susceptible R0.12 roots responded only with the deposition of lignin and phenolic compounds incapable of impeding pathogen colonization. In this study, PRR resistance was attributed to a timely multilayered response to infection by P. cinnamomi.