What to Do About Low Fruit Set in Plums? Cause Analysis and Methods to Improve It

1 Regulating tree vigor

For excessively vigorous trees, promptly remove buds and prune upright branches, and promptly open up the branch angles; alternatively, in autumn or spring apply 3-5 g of 15% paclobutrazol per tree, or spray 150-200 times diluted paclobutrazol or PBO after petal fall. For trees that are too weak, apply base fertilizer after harvest, and promptly apply topdressing and foliar fertilizer during the growing season.

2 Pollination

2.1 Matching pollinizer trees When establishing the orchard, rationally match pollinizer trees, especially for varieties with low self-fruitfulness or inherently low pollen amounts; pollinizer trees must not be less than 30%.

2.2 Introducing bees Before flowering, release 1 box of honeybees per 0.1-0.2 hm2 of orchard, or release 200 horned mason bees per 666 m2.

2.3 Artificial supplementary pollination At full bloom, use a chicken feather duster or a towel stick to gently roll between the flowers of the main cultivar and the pollinizer varieties; or mix collected and prepared pollen with talcum powder at 1:1, put it into a cloth bag, and shake it over the main cultivar trees at full bloom for pollination; you can also insert flowering branches into a water bottle and hang it on the main cultivar trees.

In addition, spraying 300-500 times diluted borax plus 100 times diluted granulated sugar or brown sugar at flowering can increase the fruit set rate.

3 Preventing frost damage at flowering

3.1 Creating windbreak forests On the north and west sides of the orchard, create windbreak forests; the windbreak forest should preferably adopt a mixed tree-and-shrub pattern, so that a dense windbreak wall is formed below 1 m to reduce frost damage.

3.2 Pay attention to orchard site selection Low-lying land easily accumulates cold air, which aggravates frost damage. Therefore, try not to establish orchards in low-lying land.

3.3 Smudging When flowering is near, pay attention to listening to (watching) weather forecasts; when frost damage is forecast at night, prepare firewood, grass, or smoke-producing agents in advance on the windward side of the orchard, placing 1 pile every 20 m, and begin smudging when the temperature drops below 0 degrees C. Note that smoke piles should be more than 10 m away from the trees, and a dedicated person must watch over them to prevent open flames from burning the trees. Smudging is relatively effective for preventing advective frost.

3.4 Irrigation If conditions permit, sprinkler irrigation facilities can be used to intermittently spray water onto the trees on nights when frost occurs. Or carry out irrigation under the trees, which has a good effect on reducing radiation frost.

3.5 Delaying the flowering period First, during the flower bud swelling stage of plum trees, spray 500-2000 mg/L maleic hydrazide or 1%-2% limewater on the trees, which can delay flowering by 3-5 d. Second, irrigate in early spring in February when the ice begins to melt; after the surface soil freezes solid, cover the ground under the trees with wheat straw, which can also delay flowering by 3-5 d. In addition, increasing the degree of pruning in winter, combined with summer pinching, and cultivating 2-3 secondary shoots to form fruiting branches, can delay the flower bud germination period and avoid frost.

3.6 Close planting Close planting cultivation can increase the population's frost resistance. Because cold air sinks, flower buds below 1 m are prone to frost damage; therefore, leaving the trunk slightly higher can reduce frost damage to flower buds from lower layers of cold air.

3.7 Variety selection Choose varieties that flower later or are more frost-resistant.

4 Controlling the plum sawfly

At the end of full bloom, carefully spray the canopy once with 1500 times diluted chlorpyrifos or 1500-2000 times diluted fenvalerate, and spray once more after 1 week, which can basically control the damage. The plum sawfly has 1 generation per year and overwinters in the soil under the trees. At the end of full bloom, spray the ground with 500 times diluted chlorpyrifos or scatter phoxim powder, then shallow hoe and rake after scattering. Pick up all fallen infested fruits from the ground cleanly and transport them out of the orchard for destruction, which can eliminate most plum sawflies.