Distribution of Alternaria leaf blight of sunflowers caused by Alternaria alternata in South Africa

Alternaria leaf blight (ALB) has been shown in recent years to be one of the major potential disease threats of sunflower capable of causing yield losses in all major production areas. The aim of this study was to determine the causal agent, prevalence and geographical distribution of ALB in the major sunflower production areas of South Africa. Surveys were conducted during 2012/13, 2013/14 and 2014/15 growing seasons at commercial sunflower production fields and at commercial cultivar trials. In the three growing seasons, twenty-nine sunflower commercial production sites were surveyed for ALB disease severity. Furthermore, four cultivars (AGSUN8251, PHB65A25, SY4200 and PAN7049) were surveyed for ALB during cultivar trials in a total of 25 localities during the three growing seasons. The plants were surveyed between 90 to 120 days after planting and leaves showing ALB symptoms were collected. Alternaria alternata was identified as the primary disease-causing organism of ALB in all the fields. Wesselsbron consistently had the lowest ALB disease severity during the 2013/14 and 2014/15 growing seasons, whereas Potchefstroom had the highest disease severity in all three growing seasons. Pearson’s correlation coefficient was greatest for temperature (r=0.6 in 2012/13, r=0.71 in 2013/14 and r=0.84 in 2014/15) and disease severity in all the growing seasons. Information about the distribution of sunflower diseases is important and this survey demonstrated that A. alternata is widespread across sunflower production areas in South Africa and may result in potential yield losses

Seasonal variation in mycoflora associated with asymptomatic maize grain from small-holder farms in two provinces of South Africa

Seed quality plays an important role in the establishment of healthy crop stands. The aim of this study was to identify the mycoflora associated with maize grain collected over two growing seasons, one experiencing severe drought, from small-holder farms across KwaZulu-Natal (KZN) and the Eastern Cape (EC), two important provinces with maize producing small-holder farmers in South Africa. Asymptomatic maize ears were collected at harvest during two maize growing seasons from farms located in Hlanganani (KZN), Ntabamhlophe (KZN), KwaNxamalala (KZN), Bizana (EC) and Tabankulu (EC). Maize grain was subjected to seed health tests using the agar plate method. The percentage incidence of fungal species isolated from maize grain was determined with species identities confirmed by ITS sequencing. Nine fungal genera were identified with Fusarium species and Stenocarpella maydis the most prevalent. Fusarium verticillioides, Fusarium graminearum and S. maydis were isolated from all sites in both seasons. No fungal species exhibited a higher incidence in the drought season across all sites.  F. graminearum and S. maydis had higher incidences in the wetter season at four and three sites, respectively. F. verticillioides had a greater incidence at the EC sites, particularly the coastal Bizana site. We conclude that local factors have a greater impact than the drought season on the population structure of ear-rot pathogens.  The widespread presence of fungi that are potentially mycotoxin-producing in asymptomatic maize grain poses health risks to consumers and is worthy of further investigation

A survey of brassica vegetable smallholder farmers in the Gauteng and Limpopo provinces of South Africa

Gedruckte Ausg. im Verlag Kassel Univ. Press (www.upress.uni-kassel.de) erschienen

Benefits of maize resistance breeding and chemical control against northern leaf blight in smallholder farms in South Africa

Maize underpins food security in South Africa. An annual production of more than 10 million tons is a combination of the output of large-scale commercial farms plus an estimated 250 000 ha cultivated by smallholder farmers. Maize leaves are a rich source of nutrients for fungal pathogens. Farmers must limit leaf blighting by fungi to prevent sugars captured by photosynthesis being ‘stolen’ instead of filling the grain. This study aimed to fill the knowledge gap on the prevalence and impact of fungal foliar diseases in local smallholder maize fields. A survey with 1124 plant observations from diverse maize hybrids was conducted over three seasons from 2015 to 2017 in five farming communities in KwaZulu-Natal Province (Hlanganani, Ntabamhlophe, KwaNxamalala) and Eastern Cape Province (Bizana, Tabankulu). Northern leaf blight (NLB), common rust, Phaeosphaeria leaf spot, and grey leaf spot had overall disease incidences of 75%, 77%, 68% and 56%, respectively, indicating high disease pressure in smallholder farming environments. NLB had the highest disease severity (LSD test, p<0.05). A yield trial focused on NLB in KwaZulu-Natal showed that this disease reduced yields in the three most susceptible maize hybrids by 36%, 71% and 72%, respectively. Eighteen other hybrids in this trial did not show significant yield reductions due to NLB, which illustrates the progress made by local maize breeders in disease resistance breeding. This work highlights the risk to smallholder farmers of planting disease-susceptible varieties, and makes recommendations on how to exploit the advances of hybrid maize disease resistance breeding to develop farmer-preferred varieties for smallholder production. Significance: Northern leaf blight, grey leaf spot, Phaeosphaeria leaf spot and common rust diseases were widespread in KwaZulu-Natal and Eastern Cape smallholder maize fields where fungicides were not applied. NLB was the most severe maize leaf disease overall. NLB caused maize leaf blighting, which reduced grain yields by 36–72% in susceptible maize hybrids. Maize resistance breeding has produced locally adapted hybrids that do not have significant yield losses under NLB disease pressure

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

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.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