J. E. van der Waals

Climate Change and Potato Production in Contrasting South African Agro-Ecosystems 3. Effects on Relative Development Rates of Selected Pathogens and Pests

A set of daily weather data simulations for 1961 to 2050 were used to calculate past and future trends in pest and disease pressure in potato cropping systems at three agro-ecologically distinct sites in South Africa: the Sandveld, the Eastern Free State and Limpopo. The diseases and pests modelled were late blight, early blight and brown spot, blackleg and soft rot, root-knot nematodes and the peach-potato aphid Myzus persicae (as indicator of Potato virus Y and Potato leaf roll virus). The effects of climate on trends in relative development rates of these pathogens and pests were modelled for each pathogen and pest using a set of quantitative parameters, which included specific temperature and moisture requirements for population growth, compiled from literature. Results showed that the cumulative relative development rate (cRDR) of soft rot and blackleg, root-knot nematodes and M. persicae will increase over the 90-year period in the areas under consideration. The cRDR of early blight and brown spot is likely to increase in the wet winter and wet summer crops of the Sandveld and Eastern Free State, respectively, but remains unchanged in the dry summer and dry winter crops of the Sandveld and Limpopo, respectively. Climate change will decrease the cRDR of late blight in all of the cropping systems modelled, except in the wet winter crop of the Sandveld. These results help to set priorities in research and breeding, specifically in relation to management strategies for diseases and pests.

Influence of environmental factors on field concentrations ofAlternaria solani conidia above a South African potato crop

Trends in weather variables and concentrations of airborne conidia ofAlternaria solani were monitored in a potato field in South Africa during three potato-growing seasons in 2001 and 2002. Distinct seasonal variation was noted, with a drop in spore numbers during winter. Peaks in spore concentration coincided with periods favorable for spore formation and dispersal; most notable was the effect of interrupted wetting periods. Diurnal periodicity of spore dispersal was also observed, with the peak of spore concentrations between 9h00 and 18h00. Few spores were sampled at night, when wind velocity and temperature are lowest and relative humidity is highest. Increased numbers of spores were sampled during days of harvesting or when other ground-operated farm equipment was used. The results obtained in this study will be useful in establishing decision support systems to control early blight on potatoes in southern Africa.

Evaluation of PLANT-Plus, a decision support system for control of early blight on potatoes in South Africa

Abstract Early blight is a major foliar disease of potatoes in most potato growing regions of the world. Numerous forecasting models have been developed and evaluated for the control of early blight on potatoes; however, prior to the development of the PLANT-Plus early blight forecaster none were applicable to South African conditions and cultivars. Field trials, conducted in spring and fall at one location and in fall at another, were used to evaluate the model. These trials compared the efficacy of fungicide programmes applied according to PLANT-Plus with a conventional calendar-based spray programme for control of early blight in South Africa. The PLANT-Plus early blight decision support system reduced the number of fungicide applications without negatively affecting yield or returns. Area under the disease progress curve-values were not significantly different for the PLANT-Plus and conventional treatments, but both were significantly lower than those of the unsprayed control. The PLANT-Plus early blight forecasting model holds great potential for use in the South African potato industry, but must be incorporated into the existing PLANT-Plus late blight forecaster in order to be more cost-effective and have multipurpose applicability.

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.

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.