Preliminary evaluation of guava selections for guava wilt disease resistance in South Africa

Guava wilt disease (GWD), caused by Nalanthamala psidii, is a serious disease occurring in the guava-producing areas of the Mpumalanga and Limpopo provinces of South Africa. Two resistant guava rootstocks, TS-G1 and ‘TS-G2’, were developed by the ARC-ITSC in 1995. In 2009, a renewed outbreak of GWD was reported, which also affected the resistant ‘TS-G2’ cultivar, placing the guava industry under threat again. The aim of this study was to seek resistant guava selections by means of in vitro screening of guava seedlings and subsequently testing the most promising selections in inoculation studies with N. psidii. A culture filtrate of N. psidii was used to screen guava seedlings in vitro. Promising selections were multiplied in tissue culture, hardened-off and planted in bags before inoculation with the GWD fungus in a shadehouse trial. The number of plants surviving nine months after inoculation was recorded. Although none of the selections showed complete resistance, selection MS44 showed some tolerance against the G2 isolate of the pathogen obtained from diseased ‘TS-G2’ trees, whilst selection MS70 showed some tolerance against the G1 isolate obtained from diseased TS-G1 trees. These selections were also resistant to the original Fan Retief isolate of the pathogen.http://www.tandfonline.com/loi/tjps20hb201

Efficacy of fungicides, plant resistance activators and biological control agents against guava wilt disease caused by Nalanthamala psidii

Guava wilt disease (GWD) caused by the fungus Nalanthamala psidii remains a major constraint to guava production in South Africa and South East Asia. In the current study, chemical and biological products as well as plant resistance activators were evaluated for control of GWD in shadehouse and glasshouse trials. In all trials, one-year-old ‘TS-G2’ guava plants were used. Plants were inoculated with a macerated culture suspension of a mixture of three isolates of N. psidii after artificial wounding of the roots. Products were applied as a soil drench or as a full cover spray. In trial 1 plants were evaluated according to a disease severity scale. In trial 2 and 3 data were recorded as number of dead plants at the termination of the trial. None of the chemical treatments caused a significant suppression of the disease. The best control was achieved with the combination of rhizobacterial strains Bacillus cereus S7 and Paenibacillus alvei T29 resulting in 53.4% and 50% disease control in trials 2 and 3 respectively. This treatment also seems to have a plant growth enhancing effect apart from disease suppression. This is to our knowledge the first report of control of GWD by means of bacterial antagonists.http://www.tandfonline.com/loi/tjps202017-11-30hb2017Microbiology and Plant Patholog

Characterisation of Mycosphaerella species associated with pink spot on guava in South Africa

Pink spot symptoms on guava fruit in the Lowveld region were in the past attributed to Colletotrichum gloeosporioides, but recently Mycosphaerella species were suggested to be part of a disease complex, including pink spot symptoms. During routine surveys of guava diseases in the Lowveld area of the Mpumalanga Province in South Africa, Mycosphaerella species were consistently isolated from guava fruit. Colletotrichum gloeosporioides was also retrieved, especially from older, bigger lesions. The Mycosphaerella isolates were compared based on their growth characteristics in culture and on DNA sequences of the internal transcribed spacer region, large subunit of the ribosomal DNA as well as the β-tubulin and translation elongation factor 1α gene regions. The phylogenetic analyses indicate that the isolates from the present study represent at least three species not previously reported on guavas. This report is therefore the first report of Mycosphaerella species associated with Psidium guajava in South Africa

Phomopsis husk rot of macadamia in Australia and South Africa caused by novel Diaporthe species

Phomopsis husk rot (PHR) in macadamia is a disease of economic importance in major commercial production areas in Australia and South Africa. Effective control of PHR is hindered by limited knowledge about its aetiology and epidemiology. The diversity and pathogenicity of more than 50 isolates of Diaporthe associated with PHR in macadamia orchards in Australia and South Africa was assessed. Multilocus phylogenetic analyses of DNA sequences of the ITS, tef1α, and tub2 gene loci revealed four novel clades that are described as Diaporthe australiana sp. nov., D. drenthii sp. nov., D. macadamiae sp. nov., and D. searlei sp. nov. Pathogenicity tests with representative isolates found that all four species caused PHR of varying severity between and within species, as well as between the two macadamia cultivars HAES 344 and HAES 816. The Australian species, D. australiana, was the most aggressive species compared with the three South African species. This study improves our understanding of the aetiology of PHR in macadamia and paves the way for more effective disease management