They may involve direct injury or abnormal growth or both. Some causes are obvious, some are obscure and many have no controls. Some environmental problems may affect all vegetables, while others only affect one crop, one variety or sometimes only 1 or 2 plants. Major Causes of Crop Injury, Poor Growth or Low Yields: 1. Diseases (fungal, bacterial, viral, phytoplasma and nematodes), insects, mites, slugs, animals and birds.
2. Environmental factors - temperature extremes, moisture extremes, wind or hail.
3. Soil Conditions - depth, texture (sand, silt, clay), drainage, structure, Alkalinity (pH), fertility and salinity.
4. Human errors or mistakes.
A. Preplanting considerations - crop and variety selection, planting dates, transplant size and condition and seed viability.
B. Cultural - soil preparation and tillage, planting techniques, plant spacing, weed control, pruning and crop interactions.
C. Chemicals - spray damage, herbicides, fertilizer burn.
D. Harvesting - timing, techniques.
5. Natural - lack of pollination, genetic (variety), environmental interactions, multiple factor problems.
Most problems are not cause by pathogenic diseases or insects, but are environmental or human problems. That means sprays are not effective in controlling the problem. Never spray unless you have identified the problem and know that the pesticide will solve the problem, not make it worse.