The role of secretion systems and small molecules in soft-rot enterobacteriaceae pathogenicity

Soft-rot Enterobacteriaceae (SRE), which belong to the genera Pectobacterium and Dickeya, consist mainly of broad host-range pathogens that cause wilt, rot, and blackleg diseases on a wide range of plants. They are found in plants, insects, soil, and water in agricultural regions worldwide. SRE encode all six known protein secretion systems present in gram-negative bacteria, and these systems are involved in attacking host plants and competing bacteria. They also produce and detect multiple types of small molecules to coordinate pathogenesis, modify the plant environment, attack competing microbes, and perhaps to attract insect vectors. This review integrates new information about the role protein secretion and detection and production of ions and small molecules play in soft-rot pathogenicity

Epidemiology of early blight on potatoes in South Africa

Early blight (Alternaria solani Sorauer)is a major foliar disease of potatoes in most growing regions of the world and is underestimated in South Africa. This project studies the epidemiology and control of the disease in South Africa. A decision support system (DSS) for early blight in South Africa was developed and evaluated in field trials. This early blight DSS is the first such system to be developed in South Africa and once incorporated with the late blight model, will represent innovative technology for use in the South African potato industry. Trends in weather variables and concentrations of airborne conidia of A. solani were monitored. Distinct seasonal variation was noted. Peaks in spore concentration coincided with periods favourable for spore formation and dispersal; most notable was diurnal periodicity and interrupted wetting periods. The results obtained from these measurements will be useful in improving early blight DSSs for southern Africa. Isolates of A. solani from various potato-growing regions in South Africa were characterized using virulence, vegetative compatibility (VC) and random amplified microsatellite (RAMS) primers. Neither the virulence assays nor VC tests sufficiently characterised the population. Analysis of RAMS profiles revealed 27% genetic diversity among the isolates. This value is similar to diversity values obtained by previous authors studying A. solani, however, it is relatively high for an asexually reproducing fungus. There was no evidence for geographical clustering of isolates, indicating that isolates are widespread across South Africa. A survey on control practices and grower perceptions of early blight in South Africa was conducted using a questionnaire. These questionnaires were distributed to growers from 10 potato-growing regions in South Africa. Results highlighted the most popular control methods and cultivars in the South African potato industry. The majority of respondents indicated that they would use an accurate, cost-effective early blight DSS, and that more research is necessary on early blight in South Africa. A survey on control practices and grower perceptions of early blight in South Africa was conducted using a questionnaire. These questionnaires were distributed to growers from 10 potato-growing regions in South Africa. Results highlighted the most popular control methods and cultivars in the South African potato industry. The majority of respondents indicated that they would use an accurate, cost-effective early blight DSS, and that more research is necessary on early blight in South Africa. Estimated crop losses ranged from 1% - 60%, with an average of approximately 20%. This is the first comprehensive epidemiological study to be conducted on early blight in South Africa and has highlighted the need for further research.Thesis (DPhil (Plant Pathology))--University of Pretoria, 2006.Microbiology and Plant Pathologyunrestricte

Emerging potato pathogens affecting food security in southern Africa: Recent research

Potato is a staple crop that contributes to food security and poverty alleviation in developing nations. Despite this, yields in developing nations are often unsustainably low, due to various biotic and abiotic factors that negatively affect production. Some of the most important biotic constraints are pathogens, many of which are disseminated by seed tubers. The lack of functional or formal seed certification systems in many southern African countries results in a continual increase in pathogen pressure. Short rotation cycles, poor plant nutrition and inefficient control measures exacerbate the crop production challenges faced by resource poor growers. In this review, we discuss five of the most important diseases on potatoes in southern Africa, namely late blight, bacterial wilt, soft rot / blackleg, powdery scab and zebra chip. Management options for small-scale growers are provided. Significance: Potato production in southern Africa is threatened by tuber-borne pathogens. Establishment and implementation of seed certification systems in southern African countries will increase potato yields and subsequently contribute to food security. Late blight, bacterial wilt, soft rot / blackleg and powdery scab are important emerging diseases on potatoes in southern Africa. Improved understanding of the biology of pathogens and the epidemiology of diseases will contributeto the management thereof

Potato virus Y and Potato leafroll virus management under climate change in sub-Saharan Africa

Potato has increased in importance as a staple food in sub-Saharan Africa, where its production is faced with a multitude of challenges, including plant disease development and spread under changing climatic conditions. The economically most important plant viruses affecting potatoes globally are Potato virus Y (PVY) and Potato leafroll virus (PLRV). Disease management relies mostly on the use of insecticides, cultural control and seed certification schemes. A major obstacle in many sub-Saharan Africa countries is the availability of disease-free quality seed potatoes. Establishment and implementation of quality control through specialised seed production systems and certification schemes is critical to improve seed potato quality and reduce PVY and PLRV sources. Seed could be further improved by breeding virus-resistant varieties adapted to different environmental conditions combined with management measures tailored for smallholder or commercial farmers to specific agricultural requirements. Innovative technologies – including more sensitive testing, remote sensing, machine learning and predictive models – provide new tools for the management of PVY and PLRV, but require support for adoption and implementation in sub-Saharan Africa. Significance: Potato virus Y (PVY) and Potato leafroll virus (PLRV) are the two major potato viruses threatening profitable seed potato production. High-quality seed shortage in many sub-Saharan Africa countries has been identified as a constraint to increasing yield. Specialised seed grower or seed certification programmes should be implemented to prevent virus transmission from seed to daughter tubers. Sustainable PVY and PLRV management in seed potatoes requires specific regional approaches to growth, farming and climatic conditions. Future research should include predictive models and new innovative technologies such as more sensitive testing, machine learning and remote sensing

Detection and eradication of Spongospora subterranea in mini-tuber production tunnels

Powdery scab, a root and tuber disease caused by the pathogen Spongospora subterranea f.sp. subterranea (Sss), poses a major problem to potato producers worldwide because it affects potato quality. Inoculum can be seed-borne or originate from contaminated growing media or contaminated equipment. During 2006, a potato mini-tuber production facility in Ceres in the Western Cape Province of South Africa had an outbreak of powdery scab. The purpose of this study was to detect Sss in the production facility and identify the source or sources of contamination so that corrective measures could be taken to eradicate the pathogen. Swab samples were taken from numerous points in the facility in 2009 and Sss-specific primers (Sps1 and Sps2) were used in a polymerase chain reaction to detect Sss. Of 11 surfaces tested, 6 were positive for Sss. A second set of swab samples was taken after efforts were made to eradicate the pathogen through improved facility hygiene measures to determine whether these corrective measures were efficient. Corrective measures resulted in a disease-free harvest from 2009 onwards. This novel study has value for the mini-tuber industry as production tunnels can be tested for the presence of Sss and other pathogens before planting to ensure that, where suitable control measures are available, disease-free mini-tubers are produced

Detection and eradication of <i>Spongospora subterranea</i> in mini-tuber production tunnels

Powdery scab, a root and tuber disease caused by the pathogen Spongospora subterranea f.sp. subterranea (Sss), poses a major problem to potato producers worldwide because it affects potato quality. Inoculum can be seed-borne or originate from contaminated growing media or contaminated equipment. During 2006, a potato mini-tuber production facility in Ceres in the Western Cape Province of South Africa had an outbreak of powdery scab. The purpose of this study was to detect Sss in the production facility and identify the source or sources of contamination so that corrective measures could be taken to eradicate the pathogen. Swab samples were taken from numerous points in the facility in 2009 and Sss-specific primers (Sps1 and Sps2) were used in a polymerase chain reaction to detect Sss. Of 11 surfaces tested, 6 were positive for Sss. A second set of swab samples was taken after efforts were made to eradicate the pathogen through improved facility hygiene measures to determine whether these corrective measures were efficient. Corrective measures resulted in a disease-free harvest from 2009 onwards. This novel study has value for the mini-tuber industry as production tunnels can be tested for the presence of Sss and other pathogens before planting to ensure that, where suitable control measures are available, disease-free mini-tubers are produced

Taxonomy and epidemiology of Pectobacterium and Dickeya spp. in Europe, North America and South Africa

&lt;p&gt;The blackleg-soft rot-aerial stem rot disease complex causes serious losses to the potato industry. It is caused by species of the genera &lt;i&gt;Pectobacterium&lt;/i&gt; and &lt;i&gt;Dickeya&lt;/i&gt;, collectively known as the soft rot Pectobacteriaceae. These soft rot Pectobacteriaceae also cause damage in a wide range of other host plants. &lt;i&gt;Pectobacterium brasiliense&lt;/i&gt; has been the most prevalent in potato and pathogenic species in Europe and South Africa for the past decade, although the species composition is in constant flux due to the introduction of new species and taxonomic reclassification of current ones.&lt;/p&gt;&lt;p&gt;Updated information on the current species composition is required, as well as knowledge of possible differences in symptom expression between species. Such information would aid certification and diagnostic services in testing for the correct species and making accurate diagnoses.&lt;/p&gt;&lt;p&gt;Findings indicate that &lt;i&gt;Pectobacterium brasiliense&lt;/i&gt; remains the most prevalent and widely distributed species in potato production areas. Other species that were identified included e.g. &lt;i&gt;Pectobacterium carotovorum&lt;/i&gt;, &lt;i&gt;Pectobacterium parmentieri&lt;/i&gt;, &lt;i&gt;Dickeya chrysanthemi&lt;/i&gt; and &lt;i&gt;Pectobacterium versatile&lt;/i&gt;. &lt;i&gt;Pectobacterium brasiliense&lt;/i&gt; was also the most pathogenic species on potato. When looking at other host plants a wide variety of &lt;i&gt;Pectobacterium&lt;/i&gt; and &lt;i&gt;Dickeya&nbsp;&lt;/i&gt;species with large genetic variation occurs.&lt;/p&gt;&lt;p&gt;MALDI-TOF MS can only be used to identify &lt;i&gt;Pectobacterium&lt;/i&gt; and &lt;i&gt;Dickeya&nbsp;&lt;/i&gt;isolates at the genus level but preliminary results after improving the reference library look promising.&lt;/p&gt;&lt;p&gt;Given that there is also a large group of nonvirulent &lt;i&gt;P. brasiliense&lt;/i&gt; isolates, a specific PCR which can differentiate between virulent and nonvirulent isolates of this species is being developed.&lt;/p&gt