Tuesday, 16 February 2021
Scott Raffle explains how our tool helps blackcurrant growers to estimate when gall mite is likely to emerge from overwintering buds.
Blackcurrant gall mite (Cecidophyopsis ribis), colonises overwintering buds on the bushes, causing the buds to swell up and shrivel. Newly infested buds gradually change shape during the summer, and by autumn they are recognisable as galls or 'big buds', being rounded, much less pointed than normal buds and usually swollen. In early spring the galled buds swell further and begin to break, but they fail to open normally and do not produce flowers or leaves. Instead, they gradually dry up and discolour, resulting in a reduction in growth, flower production and subsequent yield.
The mites act as a vector for reversion virus. Reverted bushes grow vigorously, but their cropping ability becomes seriously impaired, and they are often grubbed prematurely. Controlling gall mite is key to reducing virus spread, but this can be difficult, as the mites are confined to the buds between July and March. Except for the brief interval between leaving a gall and entering a new bud, a mite usually lives entirely within a bud.
Control is only possible between March/April and June when mites emerge from the galled buds and migrate to new unaffected buds, but experience and research have shown the timing of sprays is critical for effective control.
Forecasting gall mite migration
NIAB EMR developed a temperature sum model for forecasting the emergence of blackcurrant gall mites from galls in spring in the 1990s. The model accumulates degree-day sums above a threshold temperature of 4°C, calculated from daily maximum and minimum air temperatures by the triangulation method, starting from 15 February (Julian day 46).
AHDB uses this data in the model to predict the first 5% and 50% emergence dates for gall mite in each of the UK’s blackcurrant growing regions. The results are used to create line graphs which identify the optimum dates to apply sprays to coincide with these emergence dates. The 100 mark on the graph represents when the first mites will emerge and has shown to be accurate to within three days over the last decade. Growers are normally advised to apply control treatments just before the line reaches this mark. When the line transitions to the 150 mark on the graph, it identifies when 50% of the mites have emerged. Growers have generally aimed to apply their second control treatment just ahead of that time.
The model is run using meteorological data from key weather stations in the key blackcurrant growing regions. Growers can click on the station nearest their own geographical location to identify the optimum time to apply control measures for their own plantations.
Scott has worked for AHDB for 11 years, having spent three years at HDC and 30 years in the fruit industry in total. Prior to his time at AHDB, Scott was a fruit advisor/agronomist for 16 years with ADAS, specialising in soft fruit production and apple and pear storage.