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

Abstract

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.

Population genomic analyses suggest recent dispersal events of the pathogen Cercospora zeina into East and Southern African maize cropping systems

“…In the current study, the production practices differ between East and Southern Africa with continuous maize production in East Africa in contrast to a single summer season in South Africa. Furthermore, farmers plant a variety of genotypes, such as open pollinated varieties as well as hybrids in Kenya and Uganda, in contrast to South Africa where hybrids dominate ( Berger et al 2020 ).…”

“…Maize breeding in East Africa has generally been carried out by National programmes and institutes of the Consultative Group in International Agriculture (CGIAR) such as CIMMYT and IITA, and has made use of tropical maize germplasm with a focus on white maize for human consumption ( Worku et al 2020 ). In contrast, maize production in South Africa and Zimbabwe has been built on breeding with sub-tropical maize germplasm by local companies, national programs, and multinationals ( Berger et al 2020 ). Much of the production in South Africa is yellow maize for animal feed.…”

Deep Learning Diagnostics of Gray Leaf Spot in Maize under Mixed Disease Field Conditions

“…The main finding from this study was the development of a CNN named GLS_net, which could identify GLS disease symptoms on maize leaf disease images at an accuracy of 73.4%. Importantly, this accuracy was achieved from field images with symptoms of mixed diseases common in sub-Saharan Africa [ 45 ]. The main diseases in addition to GLS, which has thin matchstick-like lesions, were NCLB which has larger cigar-shaped lesions with pointed ends, CR which is characterized by reddish-brown pustules, and PLS with white spots [ 3 , 45 , 46 ].…”

“…Importantly, this accuracy was achieved from field images with symptoms of mixed diseases common in sub-Saharan Africa [ 45 ]. The main diseases in addition to GLS, which has thin matchstick-like lesions, were NCLB which has larger cigar-shaped lesions with pointed ends, CR which is characterized by reddish-brown pustules, and PLS with white spots [ 3 , 45 , 46 ]. The GLS_net CNN was developed using a relatively small dataset of 2332 images, but augmentation was used to increase the dataset 8-fold prior to training.…”

Combination of linkage and association mapping with genomic prediction to infer QTL regions associated with gray leaf spot and northern corn leaf blight resistance in tropical maize

“…On the other hand, NCLB is characterized by long elliptical cigar-shaped lesions on leaves that are gray-green (Welz, 1998). NCLB or TLB has been reported to cause yield reductions of 36%-72% in susceptible maize genotypes (Berger et al, 2020). The high genetic diversity reported for C. zeina and E. turcicum in Kenya (Borchardt et al, 1998;Nsibo et al, 2021) could lead to recombining pathogen populations, hence posing a greater risk to the vulnerable susceptible lines (McDonald and Linde, 2002).…”