Fungicides applied at bloom may reduce fruit set in grapes
Fungicides may have unintended side effects – use them cautiously.
Control of fungal diseases in fruit crops relies on the regular application of fungicides during the growing season, especially during and right after bloom. While fungicides are aimed at fungal pathogens, there may be unintended side effects on the crop such as phytotoxicity to the foliage or a reduction in fruit set.
Negative effects of fungicide applications on fruit set and yield have been reported in studies in apple, cranberry, raspberry and strawberry. In raspberry, the fungicides captan and benomyl reduced pollen germination and drupelet set compared to the untreated control, resulting in significantly fewer drupelets per berry. In peaches and almonds, fungicides reduced pollen germination and pollen tube growth but results varied by fungicide and cultivar. Also, effects were stronger when fungicide residues on stigmas were wet than when dry. In almonds, damage to the stigma was observed as a result of certain fungicide applications.
A number of years ago, we were surprised to find significantly lower yields in Concord grapevines in a trial that had received fungicide applications at bloom compared to untreated vines. We decided therefore to conduct field experiments in which grape flower clusters at full bloom were marked and sprayed directly with fungicides. While the number of berries per cluster was initially higher at fruit set in fungicide-treated plots, up to 40 percent lower cluster weights were observed at harvest (Figure 1) which was correlated with a reduced number of berries per cluster but not weight per berry. We observed this effect two years in a row.
In Australia, poor fruit set in grapes was seen in different grape-growing regions over several seasons and was attributed to the spraying of fungicides at the time of flowering. In field trials, the effect of fungicides applied at bloom on fruit set varied among treatments, grape cultivars and seasons. For instance, iprodione and boscalid slightly reduced pollen viability whereas copper almost completely inhibited pollen germination. It is also possible that fungicides have an indirect effect on fruit set by affecting plant physiology. In a study in France, the fungicides fludioxonil and pyrimethanil, which are commonly used against Botrytis, reduced photosynthesis and affected carbohydrate partitioning in Chardonnay grapes when applied at bloom.
While further study is needed to determine the mechanism by which fungicides reduce fruit set in grapes and how different cultivars are affected, it seems advisable to exercise caution with fungicide sprays during bloom, unless you are not concerned about potential thinning of the crop. If the disease situation and weather allows it, it may be better to apply fungicides just before or after bloom to minimize any potential negative effects on yield.