Biology, epidemiology and management of leaf spot and Botrytis on blackcurrant

Leaf spot
• Over the three years of the project the apothecial state of D. ribis was not detected in overwintering leaf litter. It therefore seems likely that this is of limited importance in the perennation of D. ribis.
• New spring epidemics of leaf spot are most likely initiated by conidia produced from acervuli overwintering in leaf litter or on twigs on the bush.
• Controlled inoculation studies showed:
1. Baldwin was much more susceptible than Ben Hope.
2. Older leaves were more susceptible to conidial infection than younger leaves.
3. Increasing length of wetness duration led to increasing incidence of leaves infected.
4. The effects of temperature were inconclusive and generally very small in comparison with other factors.
5. Significant disease increase in field conditions only occurred from July onwards (2005-07).
6. From these findings, a control strategy was developed based on early season sprays, weather related sprays before harvest and post-harvest sprays.

Botrytis
• Botrytis-infected weeds in blackcurrant plantations are potential sources of inoculum, but dead shoots and mummified berries remaining on bushes from the previous harvest are probably the most important inoculum sources.
• Baldwin and Ben Hope did not differ significantly in the susceptibility of flowers to botrytis infection: most inoculated flowers were infected or abscised one week after inoculation and nearly all remaining flowers failed to develop into mature fruit.
• The two cultivars differed significantly in their responses to botrytis infection depending on the fruiting stage at the time of inoculation. Inoculation of young fruitlets resulted in nearly 50% of fruits aborted on cv. Baldwin, compared to ca. 10% on cv. Ben Hope.
• Infection of fruit was not significantly affected by the temperature and duration of wetness.
• The incidence of latent infection decreased with increasing fruit age at the time of inoculation; inoculation of fruit near harvest resulted in significantly fewer fruit aborted.
• Frequent sampling of blackcurrant fruit in an unsprayed planting of these two cultivars in both open-field and protected conditions supported the main conclusions drawn from the controlled inoculation studies: flowers are most susceptible to infection and fruit became less susceptible with age.
• From these findings, a control strategy was formulated based on inoculum reduction in late winter, protection of flowers and green fruit and rapid processing of harvested fruit to minimise rot development post-harvest.
• The results from the inoculation studies and the proposed management strategy were largely supported in a botrytis spray timing field trial conducted over two seasons. The best control of botrytis fruit rot was achieved by sprays applied during flowering and green fruit.

Inoculum reduction
• Urea applied to green leaves was effective in encouraging leaf rotting but not effective when applied to dead leaves.
• None of the treatments evaluated were effective in reducing botrytis sporulation on mummified blackcurrant berries.

The purpose of this project was to develop an integrated disease management strategy for blackcurrant leaf spot and botrytis to enable growers to improve disease control and fruit quality with minimal use of fungicides. Specific objectives were:1) To conduct a literature review on information relating to biology, epidemiology and control of diseases of blackcurrant.
2) To understand the development of blackcurrant leaf spot in relation to key weather variables for better timing and targeting of sprays.
3) To identify the main inoculum source for botrytis in plantations.
4) To identify the effect of fruit age on susceptibility to botrytis to improve spray timing for disease control.5) To develop methods for reducing overwintering disease inoculum in blackcurrant plantations.
6) To use the information obtained in this project and that from recent research on blackcurrant pests, to produce a Best Practice Guide on pest and disease control for blackcurrant growers and advisers.