Plant name(s): Fruit trees
Symptoms / Characteristics:
There are many reasons why a plant might shed its fruit prematurely. Don't panic. Fruit drop may be natural, environmental or pest-related. Take careful observations and evaluate all possibilities. Use a process of elimination to determine the cause of the symptom and decide on an appropriate control measure.
In many cases, apple in particular, the plant undergoes certain growth phases in which natural fruit drop occurs. An early summer fruit drop commonly occurs in apple, pear and, less frequently, cherry. This is a result of the plant's inability to support the vast number of fruit that it has produced. Profuse flowering and extensive pollination result in the overproduction of fruit, beyond what the plant can physiologically sustain. In an effort to conserve energy, the plant drops the fruit. Essentially, it is a natural thinning that results from the competition between fruits.
Premature fruit drop is often related to unfavourable environmental conditions, such as late frosts, excessive heat or cold, and abrupt changes in humidity. Symptoms may be soil related, resulting from irregular watering and improper nutrition. Nutrient deficiency is a common problem. Boron-deficient green peppers, for example, will even exhibit a certain amount of fruit drop. Although there are characteristic deficiency symptoms associated with each nutrient, plant expression may vary between species. Deficiency diagnosis is further complicated if more than one nutrient is deficient in the soil. In Manitoba, only nitrogen, phosphorus, potassium and sulphur are of particular concern with respect to fruit production. Herbicide drift may also lead to premature fruit drop.
Pathological or pest-related fruit drop is more likely to occur late in the growing season when the fruit is nearing maturity. Common insects that cause premature fruit drop include apple maggot and plum curculio. Common diseases include apple scab and peach leaf curl. Insects and diseases tend to have more visually identifiable symptoms and are, therefore, easier to diagnose than environmental or physiological disorders.
Control / Preventions:
Thin fruit to reduce competition and encourage the plant to put more energy into producing fewer numbers of larger, higher quality fruit. The removal of fruit beyond what is lost during the early season drop may even be necessary. Some horticulturists even suggest thinning the blossoms, but flowers are typically an attractive feature for most homeowners. Avoid unfavourable environmental conditions that might cause a plant to drop its fruit. This involves effective water management and a balanced fertilizer program, according to individual plant specifications. Soil testing may be required in order to confirm nutrient deficiency/toxicity. Supplement with fertilizer where necessary. Avoid herbicide drift. Never apply herbicides in windy or dead calm conditions. Contrary to popular belief, dead calm conditions are often associated with a phenomenon known as temperature inversions. Spraying under such conditions can actually increase drift distance. If additional symptoms are observed on fruit, leaves or stems, proceed to identify the causal agent and administer appropriate control measures.
Relevant web sites:
Jones, A.L. and Sutton, T.B. 1984. Diseases of Tree Fruits. Cooperative Extension Service, Michigan State University. 59 pages.
Manitoba Agriculture and Food Fruit Guide 2000 Edition. 262 pages.