Physiological plant disorders are caused by non-pathological conditions such as poor light, adverse weather, water-logging,phytotoxic compounds or a lack of nutrients, and affect the functioning of theplant system. Physiological disorders are distinguished fromplant diseases caused bypathogens, such as avirus orfungus.^[1] While the symptoms of physiological disorders may appear disease-like, they can usually be prevented by altering environmental conditions. However, once a plant shows symptoms of a physiological disorder, it is likely that that season's growth or yield will be reduced.

Diagnosis of the cause of a physiological disorder (or disease) can be difficult, but there are many web-based guides that may assist with this. Examples are:Abiotic plant disorders: Symptoms, signs and solutions;^[1]Georgia Corn Diagnostic Guide;^[2]Diagnosing Plant Problems (Kentucky);^[3] andDiagnosing Plant Problems (Virginia).^[4]

Some general tips to diagnosing plant disorders:

• Examine where symptoms first appear on a plant—on newleaves, old leaves or all over?
• Note the pattern of any discolouration or yellowing—is it all over, between theveins or around the edges? If only the veins are yellow, deficiency is probably not involved.
• 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, such as determiningpH, can help to confirm the presence of physiological disorders.
• Considering recent conditions, such as heavy rains, dry spells, frosts, etc., may also help to determine the cause of plant disorders.

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Frost and cold are major causes ofcrop 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.^[5] Symptoms will often appear overnight, affecting many types of plants. Leaves and stems may turn black, and buds and flowers may be discoloured, and frosted blooms may not producefruit. Manyannual plants, or plants grown in frost free areas, can suffer from damage when the air temperature drops below 40 degreesFahrenheit (4 degreesCelsius). Tropical plants may begin to experience cold damage when the temperature is 42 to 48 °F (6 to 9 °C), symptoms includewilting of the top of the stems and/or leaves, and blackening or softening of the plant tissue.

Frost or cold damage can be avoided by ensuring that tender plants are properlyhardened before planting, 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 withhorticultural fleece if frost is forecast. Cold, drying easterly winds can also severely inhibit spring growth even without an actual frost, thus adequate shelter or the use of windbreaks is important.

Drought can cause plants to suffer from water stress and wilt. Adequateirrigation is required during prolonged hot, dry periods. Rather than shallow daily watering, during a drought water should be directed towards the roots, ensuring that the soil is thoroughly soaked two or three times a week.Mulches also help preserve soil moisture and keep roots cool.

Heavy rains, particularly after prolonged dry periods, can also cause roots to split,onion saddleback (splitting at the base),tomatoes split andpotatoes to become deformed or hollow. Using mulches or adding organic matter such asleaf mold,compost or well rottedmanure to thesoil will help to act as a 'buffer' between sudden changes inconditions. Water-logging 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.

Hail can cause damage to soft skinned fruits, and may also allowbrown rot or otherfungi to penetrate the plant. Brown spot markings or lines on one side of a matureapple are indicative of a spring hailstorm.

Plants affected bysalt stress are able to take water from soil, due to anosmotic imbalance between soil and plant.

Poor growth and a variety of disorders such as leaf discolouration (chlorosis) can be caused by a shortage of one or more plant nutrients. Poor plant uptake of a nutrient from the soil (or other growing medium) may be due to an absolute shortage of that element in the growing medium, or because that element is present in a form that is not available to the plant.^[6] The latter can be caused by incorrectpH, shortage of water, poor root growth or an excess of another nutrient.^[6] Plant nutrient deficiencies can be avoided or corrected using a variety of approaches including the consultation of experts on-site, the use ofsoil andplant-tissue testing services, the application of prescription-blendfertilizers, the application of fresh or well-decomposedorganic matter, and the use of biological systems such ascover crops,intercropping,improved fallows,^[7]ley cropping,permaculture, orcrop rotation.

Nutrient (or mineral) deficiencies include:

• Boron deficiency
• Calcium deficiency
• Iron deficiency
• Magnesium deficiency
• Manganese deficiency
• Molybdenum deficiency
• Nitrogen deficiency
• Phosphorus deficiency
• Potassium deficiency
• Zinc deficiency

• Physiological plant disorders
• Horticulture

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