It is important to be able to distinguish between plant growth problems that are caused by pathogens, such as a virus or fungus, and those which are caused by non-pathological disorders in the functioning of the plant system, such as poor light, weather damage, water-logging or lack of nutrients. Whilst the symptoms may appear disease-like, physiological plant disorders can usually be prevented by altering environmental conditions, although once a plant is showing the symptoms of a deficiency it is likely that that seasonís yields will be reduced.

Before jumping to conclusions about why a plant is failing or showing symptoms, it is worth thinking systematically in order to identify the causes of a physiological disorder:

  • Note where symptoms appear first- on the new leaves, the old leaves or all over?
  • Note the pattern of any coloration or yellowing- is it all over, between the viens or around the edges? If the veins only are yellow it is not a deficiency.
  • Note general patterns rather than looking at individual plants- are the symptoms distributed throughout a group of plants of the same type growing together. In the case of a deficiency all of the plants should be similarly effected, although distribution will depend on past treatments applied to the soil.
  • Soil analysis, eg, determining pH, can help to confirm the diagnosis. Be aware of recent conditions, eg, heavy rains, dry spells, frosts, etc. Again, this will help to determine the possible cause of problems.

The causes of physiological plant disorders can include;

Weather damage

Frost is a major cause of crop damage to tender plants, although hardy plants can also suffer if new growth is exposed to a hard frost following a period of warm weather. Symptoms will appear overnight, affecting unrelated plants. Leaves and stems may turn black, and buds and flowers be discoloured. Frosted blooms may not produce fruit.

Frost damage can be avoided by ensuring that tender plants are properly hardened off before planting out, and that they are not planted too early in the season, before the risk of frost has passed. Avoid planting susceptible plants in frost pockets, or where they will receive early morning sun. Protect young buds and bloom with horticultural fleece if frost is forecast. Cold, drying easterly winds can also severely check spring growth without an actual frost, thus adquate shelter or the use of windbreaks is important. Drought can cause plants to suffer from water stress and wilt. Adequate irrigation will be necessary in prolonged hot dry periods. watering should be directed towards the roots, ensuring that the soil is thoroughly soaked two or three times a week rather than a shallow watering daily. Mulches will also help to preserve soil moisture and keep the roots cool. Waterlogging can occur on poorly drained soils, particularly following heavy rains. Plants can become yellow and stunted, and will tend to be more prone to drought and diseases. Improving drainage will help to alleviate this problem;

Heavy rains, particularly after prolonged dry periods, can also cause roots to split, onion saddleback (splitting at the base), tomatoes to split and potatoes to become deformed or hollow. Using mulches or adding plenty of organic matter such as leaf mold, compost or well rotted manure to the soil will help to act as a 'buffer' between the sudden change of conditions. Hail can cause damage to soft skinned fruits, and may also allow brown rot or other fungi to penetrate. Brown spot marking or lines on one side of mature apples are indicative of a spring hailstorm.

Nutrient deficiencies

Poor growth and a variety of complaints such as leaf discoloration can be caused by a lack of plant foods. This may be due to shortages of necessary nutrients, or because the nutrients are present but not available to the plant. This can be caused by reasons including incorrect pH, shortages of water or an excess of another nutrient. Generally, the key to avoiding nutrient deficiencies is to ensure that the soil is healthy and contains plenty of well rotted organic matter rather than feeding or treating individual plants.

Nutrient (or mineral) deficiences include;

Shortage of trace elements such as molybdenum (Mo) can also cause disorders such as whiptail in cauliflowers.