Streptomyces species associated with common scab lesions of potatoes in South Africa

Streptomyces scabiei is largely accepted as the causal organism of common scab on potato in South Africa, and other Streptomyces species associated with common scab are not often considered. This study therefore aims to determine the diversity and prevalence of Streptomycetes associated with common scab on potatoes in South Africa. Isolates from 11 of the 16 potato producing regions in South Africa were characterized morphologically, physiologically and genetically. Most isolates resembled S. scabiei based on morphology and physiology. Most pathogenic isolates were S. scabiei and S. stelliscabiei, and no S. acidiscabies or S. turgidiscabies isolates were found. All three pathogenicity/virulence genes (txtAB, nec1, tomA) were found in South African isolates. Pathogenicity could not be linked to the presence of a single one or any combination of two of the three genes. These results represent the most comprehensive published survey of Streptomycetes isolated from common scab lesions on potatoes in South Africa.National Research Foundation of South Africa (UID: 78566 (NRF RISP grant for the ABI3500)) and University of Pretoria.http://link.springer.com/journal/106582017-03-30hb201

With drought comes charcoal rot

Charcoal rot was first reported in South Africa when found on sunflower in 1969, but had already been detected on maize and sorghum by that time. The first incidence of this disease on soya beans was reported in 1982. Maize and sorghum are frequently planted in rotation with sunflower and soya beans.http://reference.sabinet.co.za/sa_epublication/vp_oilseedsam2016Plant Production and Soil Scienc

Effects of different soil treatments on the development of Spongospora subterranea f. sp. subterranea in potato roots and tubers in the greenhouse

Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) causes extensive losses in potato production systems globally. Two pot experiments were established in the greenhouse in summer 2013 and winter 2014 to evaluate the effectiveness of different soil chemicals, fumigant, amendments and biological control agents (BCAs) against Sss in the rhizospheric soil, potato roots and tubers. The study used visual assessment methods to assess the effect of treatments on root galling and zoosporangia production, and qPCR to measure Sss concentration in the soil and in the potato roots and tubers. All six soil treatments, namely metam sodium, fluazinam, ZincMax, calcium cyanamide, Biocult and a combination of Bacillus subtilis and Trichoderma asperellum recorded significantly (P < 0.05) lower numbers of zoosporangia in the roots compared to the untreated control. The same effect was observed on the concentration of Sss DNA in the roots at tuber initiation. A more diverse picture was obtained when root gall scores at tuber initiation and Sss DNA in the rhizospheric soil at tuber initiation and harvesting were compared. Significant differences (P < 0.05) were also noted in disease severity, disease incidence, and tuber yield between metam sodium, fluazinam, ZincMax, calcium cyanamide and the untreated control. Calcium cyanamide gave the highest tuber yield. The study demonstrated the potential of soil treatments such as metam sodium, fluazinam, ZincMax and calcium cyanamide in managing Sss in potatoes by reducing the pathogen both in the rhizospheric soil and the roots of the potato plant.The National Research Foundation (NRF), South Africa, and Potatoes South Africa.http://link.springer.com/journal/115402018-03-30hj2017Plant Production and Soil Scienc

A search for anthracnose resistant cashew cultivars in Mozambique

Dwarf and common cashew (Anacardium occidentale) genotypes were screened separately for resistance against anthracnose (Colletotrichum gloeosporioides). Disease incidence was assessed on emerging leaves over three consecutive crop seasons in Mocuba, Meconta and Pebane districts of northern Mozambique. Evaluation the disease using leaf incidence is presented as a new field method for screening cashew genotypes resistant to anthracnose. It is fast, precise and consistent in ranking cultivars over several tree seasons. Seasonal, cultivar and disease incidence means were compared using Fishers’ LSD test. The method enabled the differentiation of highly infected cultivars from those consistently tolerant across seasons and locations. No a single clone with a high level of resistance was identified out of 229 entries. However, hierarchical tables of clonal sensitivity ranked clones 1.12PA, 12.8PA and 1.18PA as tolerant and 11.9PA and 2.3BG as susceptible among the dwarfs. Among the common genotypes, clones NA7, MB77, 1.5R and MCH-2 ranked tolerant and IM1 and MU3 susceptible. Tolerant clones were therefore recommended to be used in the national cashew breeding program for further development of cashew cultivars with durable resistance to anthracnose. Further, clones such as 2.5VM, 1EM, MB75 and others that revealed incidence consistency over seasons can be used as susceptibility or tolerance standards in screening trials.AFD (French Development Agency, Mozambique) was funding this research through the PRC/PIAC-Project, Nampula.www.elsevier.com/locate/croprohb201

Epidemiology of cashew anthracnose (Colletotrichum gloeosporioides Penz.) in Mozambique

Anthracnose of cashew (Anacardium occidentale) was studies on various genotypes and locations in Mozambique. Colletotrichum gloeosporioides was identified as the anthracnose causal agent using polymerase chain reaction. The relationships between incidence and severity of anthracnose on cashew genotypes were statistically analyzed by regression. Anthracnose leaf incidence, which is practically easy to evaluate, was consistently associated with leaf severity, and their relationships can be estimated using the restricted exponential function across locations, crop seasons, genotype and fungicide trials. Pooled data enabled estimation of initial incidence of 1.43% with percentage variance accounting for 83.2 and standard error of 8.3. By computing incidence data into the summary equation, 24 changes of 0, 1, 5, 10 and 40%, resulted in changes of severity estimates of 0.01, 0.05, 0.10, 0.50 and 1.00%, respectively. The maximum disease incidence was estimated as 80% when the severity reached only 5%. Increase in severity was observed afterward, approached a maximum of 25% when leaf detachment is observed. The use of incidence data for epidemic comparisons, genotype and fungicide evaluation in cashew orchards is recommended. Anthracnose incidence on leaves however, could not predict incidence on nuts.The AFD (French Development Agency, Mozambique) funded this research through PRC/PIAC-Project.www.elsevier.com/locate/croprohb201

First Report of soft rot of potatoes caused by Dickeya dadantii in Zimbabwe

A survey was carried out in the potato- (Solarium tuberosum L.) growing regions of Zimbabwe in April 2009 to assess the prevalence of bacterial soft rot. A total of 125 tubers with soft rot symptoms were collected. The disease caused severe economic losses ranging from 20 to 60% on tubers in the field and in storage.http://www.apsnet.org/nf201

Pathogenicity of fungi isolated from atypical skin blemishes on potatoes in South Africa and Zimbabwe

Potato skin blemishes with known causes (typical blemishes) are manageable in contrast to those with unknown causes (atypical blemishes). This study investigated possible fungal causal agents of atypical tuber blemishes in South Africa and Zimbabwe through isolation and pathogenicity testing. Several fungal genera were isolated from elephant hide, corky cracks, star-shaped lesions, circular corky lesions, raised corky spots, enlarged lenticels and russetting. Koch’s postulates could not be fulfilled with most of the isolates except for binucleate Rhizoctonia (BNR) AG A, Macrophomina phaseolina and Rhizoctonia solani AG 3-PT. BNR AG A isolates from South African tubers of cv. Up-to-Date reproduced circular and linear defects on tubers. M. phaseolina isolates inoculated on South African cv. Mondial reproduced dark, circular lesions around lenticels conforming to typical symptoms of charcoal rot on potato tubers. R. solani AG 3-PT isolates reproduced corky cracks with elephant hide and black scurf with similar aggressiveness on cvs Mondial (South Africa) and Diamond (Zimbabwe). This is the first report of charcoal rot caused by M. phaseolina and of tuber defects caused by BNR AG A on potato in South Africa. This is also the first report of tuber cracking and elephant hide caused by R. solani AG 3-PT on potato in Zimbabwe. The findings from this work are crucial in formulating control strategies against these pathogens in order to improve potato tuber quality on the market.The National Research Foundation (NRF)http://link.springer.com/journal/115402018-06-29hj2017Forestry and Agricultural Biotechnology Institute (FABI)Microbiology and Plant PathologyPlant Production and Soil Scienc

Anastomosis groups and pathogenicity of Rhizoctonia solani and binucleate Rhizoctonia from potatoes in South Africa

A survey of anastomosis groups (AGs) of Rhizoctonia species associated with potato diseases was conducted in South Africa. A total of 112 Rhizoctonia solani and 19 binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants, characterized for AG and pathogenicity. The AG identity of the isolates was confirmed using phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. Rhizoctonia solani isolates recovered belonged to AG 3-PT, AG 2- 2IIIB, AG 4HG-I, AG 4HG-III and AG 5, while BNR isolates belonged to AG A and AG R, with frequencies of 74, 6.1, 2.3, 2.3, 0.8, 12.2 and 2.3%, respectively. Rhizoctonia solani AG 3-PT was the most predominant AG and occurred in all the potato growing regions sampled whereas the other AGs occurred in distinct locations. Different AGs grouped into distinct clades with high maximum parsimony and maximum likelihood bootstrap support for both R. solani and BNR. An experiment under greenhouse conditions with representative isolates from different AGs showed differences in aggressiveness between and within AGs. Isolates of AG 2-2IIIB, AG 4HG-III and AG R were the most aggressive in causing stem canker while AG 3-PT, AG 5 and AG R caused black scurf. This is the first comprehensive survey of R. solani and BNR on potatoes in South Africa using a molecularbased approach. This is the first report of R. solani AG 2-2IIIB and AG 4 HG-I causing stem and stolon canker and BNR AG A and AG R causing stem canker and black scurf on potatoes in South Africa.Potatoes South Africa.National Research Foundation of South Africa (UID: 78566 (NRF RISP grant for the ABI3500)).http://apsjournals.apsnet.org/loi/pdishb201

Population genetic structure of Rhizoctonia solani AG 3-PT from potatoes in South Africa

Rhizoctonia solani AG 3-PT is an important potato pathogen causing significant yield and quality losses in potato production globally. However, little is known about the levels of genetic diversity and population structure of this pathogen in South Africa. A total of 114 R. solani AG 3-PT isolates collected from four geographic regions were analyzed for genetic diversity and structure using eight microsatellite loci. Microsatellite analysis found high intrapopulation genetic diversity, population differentiation and evidence of recombination. A total of 78 multilocus genotypes (MLGs) were identified with few MLGs shared among populations. Low levels of clonality (13-39 %) and high levels of population differentiation were observed among populations. Most of the loci were in Hardy-Weinberg equilibrium and all four field populations showed evidence of a mixed reproductive mode of both clonality and recombination. The PCoA clustering method revealed genetically distinct geographic populations of R. solani AG 3-PT in South Africa. This study showed that populations of R. solani AG 3-PT in South Africa are genetically differentiated and disease management strategies should therefore be applied accordingly. This is the first study of the population genetics of R. solani AG 3-PT in potatoes in South Africa and results may help to develop knowledge-based disease management strategies in South Africa and elsewhere.Potatoes South Africa and the Potato Pathology Programme at UP as well as the National Research Foundation.http://www.elsevier.com/locate/funbio2017-05-31hb2016Plant Scienc

Cardinal temperature differences, determined in vitro, between closely related species and subspecies of pectinolytic bacteria responsible for blackleg and soft rot on potatoes

Potato blackleg and soft rot cause major losses and are caused by two bacterial genera, Pectobacterium and Dickeya. Species affecting potatoes are Pectobacterium atrosepticum (Pba), Pectobacterium carotovorum subsp. carotovorum (Pcc), Pectobacterium carotovorum subsp. brasiliense (Pcb), Pectobacterium wasabiae (Pwa), Dickeya dadantii (Dda) and Dickeya solani (Dso). Pathogenicity of these species is dependent on temperature, with each species having its own optimal temperature and temperature range for growth, leading to varying degrees of losses. Pectobacterium atrosepticum, a temperature sensitive species, mainly occurs in temperate climates, Pcc in temperate to tropical, and Dickeya spp. in subtropical environments. The aim of this study was to determine the cardinal growth temperatures for the species responsible for blackleg and soft rot in vitro. Bacterial isolates were incubated in a temperature gradient shaking incubator at 30 different temperatures ranging from ±5 °C to ±56 °C, and growth measured at two set time intervals. Results were statistically analysed using the Gaussian function. The optimal temperature of 31 °C and temperature range of 20 °C to 38 °C for Pectobacterium carotovorum subsp. brasiliense, was similar to those recorded for Pcc. Pectobacterium wasabiae grew at an optimal temperature of 29 °C and range of 20 °C to 34 °C. Higher optimal temperatures of 32 °Cand 34 °C,with ranges of 21 °C to 38 °Cand 23 °C to 41 °C were recorded for Dda and Dso, respectively. The minimal variation in optimal temperatures between different species might be an indication that temperature ranges, rather than optimal temperature, play an important role in disease development. Results for Dso, which has not yet been reported in South Africa, are especially important in light of prevailing temperatures in South African potato production regions.Technology and Human Resources in Industry Programme (THRIP) and Potatoes South Africahttp://link.springer.com/journal/106582017-02-20hb201