Developing effective integrated control measures for the control of black dot in potatoes

• laboratory studies to understand how initial inoculum on seed and in soil relates to final disease and thus disease risk
• field trials to study the epidemiology of C. coccodes under a range of inputs and control measures
• field monitoring to determine how disease develops in commercial crops
• and post-harvest trials to identify the optimum way to handle stocks after harvest to reduce infection / disease development.

In April 2007, the AHDB Potatoes funded a one year extension to the project. The aim was to provide additional information needed for the interpretation of results from the PCR-based diagnostics assay. The researchers examined the effect of the timing of sample collection on the ability to detect black dot in soil and covered a wider range of varieties and soil types than previously studied.

Results

Objective 1: Understand the relationship between source of inoculum and disease development. Seed inoculum was found to be relatively less important than soil inoculum in causing disease. This was true irrespective of the level of seed infection. Where both sources of inoculum were present, seed inoculum was found to enhance the effect of soil inoculum. In a seed stock where black dot was present, even visually disease-free tubers were found to be infected. A strong relationship was found between the level of soil inoculum and black dot on progeny tubers. Thresholds of 100 and 1000 pg DNA / g soil from soil tests have been adopted to provide guidance on medium and high risk of black dot, respectively.

Objective 2: Establish agronomy factors that affect crop status and how these influence disease. Seasonal environmental factors, particularly wetter conditions and higher temperatures, influence disease development and this should be considered when interpreting the soil test threshold. The impact of temperature and moisture was confirmed in a controlled environment experiment, by the fact that irrigation increased black dot over non-irrigation in field trials and from relating weather records to black dot development in field trials. Methods to measure crop stress were evaluated but it was not possible to relate measurements to effects on disease development.

Objective 4: Determine the optimum integration of control measures. Field trials evaluated and quantified all other principal control measures for black dot (variety resistance, irrigation, fungicide use and crop duration). The selection of a more resistant variety, even by a single disease resistance rating unit resulted in a significant reduction in incidence and severity of black dot on progeny tubers. Targeting varieties to fields according to disease risk represents a very cost effective way of limiting black dot.

The length of crop duration, from 50% emergence to harvest, showed a close relationship to final black dot levels on progeny tubers. Restricting crop duration or avoiding unnecessary delay in harvest where black dot risk is high was clearly shown to limit black dot development.

Objective 5: Improve disease forecasting from visual examination of below ground plant parts. Monitoring in field trials showed that visible disease was present on stems, stolons and roots at least 6 weeks prior to haulm destruction when the main source of inoculum was seed tubers. Thereafter, visible disease developed slowly. Where soil was the main source of inoculum, visible infection occurred later but the rate of visible disease development was more rapid. However, it was concluded that level of stem, stolon or root infection around haulm destruction could not be used as a predictor of final disease.

Objective 6: Determine the impact of post-harvest storage regime on disease development. It was demonstrated that development of black dot after harvest was inhibited to the greatest extent by an immediate pull-down in temperature to the holding temperature. In contrast, applying a curing regime led to an increase in black dot during storage. There was no difference in black dot between holding storage temperatures of 2.5^oC or 3.5^oC.

The final report from a previous project "Prediction and manipulation of black dot development in potato crops" published in 2005 is also provided below. The R400 Final Report summarises the work to determine how knowledge of crop duration and black dot soil inoculum levels can be used to predict black dot risk during storage.