Studies conducted many years ago (MacDaniels and Hildebrand, 1940) showed that use of fungicides could inhibit the germination of apple pollen on the stigma.
More recent research has shown that sprays of certain fungicides, when sprayed onto apple trees during flower opening (anthesis) can have a toxic effect on the reproductive organs in the flowers (Bonomo and Tiezzi, 1986; Fedke, 1982). It has been suggested that some of the fungicides have an adverse effect on the cytoskeletal apparatus and mitosis within the pollen tube and the maternal tissues (Battistini et al., 1990).
Recent studies have tested sprays of benomyl, dodine and penconazole at various concentrations applied to the flowers of Golden and Red Delicious.
- As shown previously, benomyl had a negative influence on pollen germination (70% reduction).
- At recommended dose rates dodine also had a severe effect on pollen germination (80% reduction). Penconazole had negligible effects on Red Delicious but negatively affected pollen germination of Golden Delicious (75% reduction at recommended rates) even when applied at one eighth the recommended dose rates.
Surprisingly, and in contrast to the above results, researchers working at Long Ashton Research Station some years ago noted increased cropping of Cox’s Orange Pippin following sprays of benomyl at or around blossom time (Byrde et al., 1976).
- This was attributable to the fungicide reducing stylar infections with various fungi (Williams et al., 1971).
- Similar increases in yield were reported in South Africa and the UK following sprays of triadimefon (Strydom and Honeyborne, 1981; Church et al., 1984).
- Work at Long Ashton also showed that in some years sprays of sulphur at green cluster or pink bud could also increase fruit set (Church and Williams, 1983).
- Sulphur may occasionally damage the primary leaves of some apple varieties and it has been suggested that the increased fruit set recorded following its use may be attributable to sulphur’s effect in checking growth of bourse shoots.
- This in turn would diminish unwanted competition between these shoots and the fruits for assimilates (the plant’s food stuffs) (Church and Williams, 1983).
- This needs further research in the light of increased use of sulphur for organic growing systems.
Research therefore indicates that sprays of certain pesticides could negatively affect the germination and growth of pollen, if they were applied to open flowers.
- The results showed that sprays of Captan, and penconazole (Topas) could all reduce the germination of pollen and influence fruit set on some apple varieties.
- The above results were often obtained using in vitro tests and are likely to exaggerate slightly the negative influences of fungicides on pollen germination in comparison with field experiences.
- Nevertheless, it is advisable not to spray these fungicides during flowering wherever possible.
- Most new fungicides are now tested for their effects on pollen germination and growth and the manufacturers should supply information on the results of these tests.
Fungicidal sprays are likely to have no measurable effect on fruit set in years when pollen is transferred in abundance by insects and weather conditions are favourable for its growth.
- However, in years unfavourable to pollen transfer and growth, they could have severely deleterious effects on fruit set (Church et al., 1976).
- The mechanisms of how these effects are brought about are still not understood.
The influence on pollination and fruit set of the growth regulators paclobutrazol (Cultar) and CPPU (a chemical occasionally used abroad which has cytokinin action) was also evaluated in trials.
- The effects of paclobutrazol were variable.
- At high rates the sprays had no effect on pollen germination whilst at the recommended rates the pollen germination of Golden Delicious was slightly stimulated and that of Red Delicious slightly depressed.
- Sprays of the growth regulator paclobutrazol (Cultar) therefore appear to have no negative effects on pollen germination.
- CPPU had no significant effect on pollen germination. Research at Long Ashton showed that sprays of paclobutrazol (1.0 ml l-1) applied to Cox trees between green cluster and petal fall reduced initial and final fruit set (Church et al., 1984).
- However, this may have been due to a negative effect of the sprays on the hormone balance within the flower, rather than any direct effect on the pollen.
Insecticides are known to have a very negative effect on bees and other insects (Johansen, 1977) and it is recommended that sprays of most insecticides are suspended during blossoming.
- However, their effects upon bee visits (i.e. the attractiveness of the flowers) and pollen germination and growth are less clear.
- Trials at Long Ashton Research Station showed that chorpyriphos (Lorsban, Equity) applied during flowering of Cox trees reduced fruit set (Church et al., 1984).
Sprays of Captan, penconazole (e.g. Topas) and chlorpyriphos (e.g. Equity and Lorsban) have been shown to reduce fruit set in some seasons if applied during flowering.
- Their effect may be due to their inhibition of pollen germination and growth, as shown by in vitro tests on some of the pesticides.
- Alternatively it may be a direct effect on the pollinating insects.
- It is suggested that, where possible, they are not applied during flowering time.