Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
As fruiting vegetables reach full size, we often get called to look at problems with fruit set. Poor fruit set, blossom drop, and fruit abortion can be caused by many factors including heat stress, water stress, insect and mite damage, chemical injury, lack of pollinating insects, and nutrient deficiencies to name few. Excessive vegetative growth is another common reason for poor fruit set or delayed fruit set.
Excessive vegetative growth commonly occurs when plants are grown under high fertility conditions and are being heavily watered. Under these conditions, flowering and fruiting is delayed as plants continue to put on new growth. High plant populations or close plant spacing can further compound the problem.
In this vegetative growth mode, the hormone balance is such that flowering and fruiting is limited. This will not change until growth slows and the hormone balance shifts, signaling flowering.
In addition, as the area covered by leaves and stems increases, so does shading within crop canopies. New growth starts to shade out older growth. Unfortunately, the stem nodes where flowering and fruiting is occurring is often so shaded that nearby leaves cannot produce enough photosynthate to support these flowers and fruits.
Heavy dense canopies can also reduce the effectiveness of pollinating insects as flowers are more difficult to access.
Another problem with excessive vegetative growth is creating a high humidity environment in the canopy that favors plant diseases. Sclerotinia (white mold), Phytophthora, Pythium, Septoria, and Botrytis are examples of diseases that can become problems in dense canopies. Often, these diseases will infect flowers and young fruit, causing them to drop.
We commonly see delayed fruiting, poor fruit quality, and reduced fruiting in vine crops with excessive growth. This includes watermelons, cantaloupes, and pumpkins. This is especially true of robust vining varieties. In contrast, bush and semi-bush forms of cucurbits such as some summer squashes and winter squashes have fewer problems with overgrowth. Tomatoes are also susceptible to excessive plant growth, reducing fruit set and quality. Snap beans and lima beans with excessive foliage are at high risk for diseases infecting pods, causing pod drop. Cucumbers with excessive foliage also are high risk for fruit diseases.
The goal in producing these fruiting vegetables is to balance vegetative and reproductive growth. This is done by managing fertility, especially nitrogen (N), to have enough nutrients available to grow a healthy plant but not to promote excessive growth. Nitrogen fertilization rates in the commercial vegetable production recommendations guide for Delaware should be consulted. In addition, nitrogen release from manures or other organic sources needs to be considered in overall N applications. We often see over-fertilization problems in fields with a history of heavy manure use or with high organic matter where N is being released from these sources as they mineralize.
Water should be managed so that plants are not over-irrigated. You certainly should not let plants become so water stressed that growth is affected. However, low levels of water stress actually can promote reproductive development in many vegetables.
Crop type, plant type, and plant morphology should also be considered prior to planting. Varieties that produce heavy growth (long vines or stems, many branches, large leaves, etc.) need to be spaced further apart to avoid overcrowding. Some vigorous varieties will even require reduction in fertilizer to control growth. Also consider how each crop responds to high fertility and heavy irrigation. Crops such as sweet corn, summer squash, and peppers normally do not have problems with excessive growth and will yield well under those conditions. In contrast, tomatoes, watermelons, cantaloupes, and lima beans will have yield reductions if fertility and irrigation are not managed well.