Gordon Johnson, Extension Vegetable & Fruit Specialist;gcjohn@udel.edu
We are in the harvest season for fruiting vegetables – watermelons, cantaloupes, tomatoes, peppers and others.
A vegetable plant’s carrying capacity for fruits will be affected by the amount of healthy, effective leaf area. Effective leaf area is that which has adequate exposure to sunlight and that is not excessively shaded. Plants generally have excess leaf area and can lose some leaf area before affecting yield. However, once a critical point is reached in lost leaf area, yield will decline. When leaf area is compromised by diseases, insect or mite damage, or injury (hail, wind, chemicals, fertilizer burn, leaf scorch), then fruit loads or fruit quality may be affected. Plants may abort fruits to carry only the number that can be supported by the photosynthates produced. Fruits may be smaller in size and quality (such as sugar content) is often reduced.
When plants are carrying full fruit loads, this also places additional stress on the plant. The plant shifts food resources to the fruits and away from other uses such as defensive chemicals used to combat diseases and insects. It is when plants have full fruit loads that they are often the most susceptible to pests. For example, we often see mite damage in watermelon pollenizers at higher levels than in seedless plants. This is because they are flowering sooner and are producing both male flowers and fruits and have heavier demands placed upon them. It is interesting to see fields of seedless watermelons where pollenizers are missing (were not planted or died). The plants are extra vigorous and healthy. This is because they have reduced fruit loads.
Overly vigorous plants will produce too much foliage and excessive shading can occur in leaves that are covered. This reduces effective leaf area and can set up conditions that favor diseases. It also keeps plants in a “juvenile” stage and delays flowering and fruiting. The goal is to strike a balance by having adequate but not excessive leaf area to support developing fruits. Attention should be paid to populations and to nitrogen levels to avoid excessive foliage.
Fruit loads are also determined by fruit set. Those environmental factors that affect fruit set such as heat and cold will reduced fruit loads. This may be due to reduced pollen production, reduced pollen viability, reduced pollen transfer, or increased abortion of flowers.