Physiological disorders. Light, water, temperature and nutrition

Non-infectious disease and secondary infection

A physiological disorder arises without the initial involvement of a pathogen, through an unsuitable supply of water, oxygen, light, temperature or mineral nutrients. However, it may damage tissues and create conditions for secondary infection. Thus, “overwatering” and “rot” are not mutually exclusive diagnoses, and persistent tissue breakdown cannot be explained by watering alone. [9-WWW] [Fleischer and Schütz 1978, pp. 183–188]

Excess light, overheating and insufficient light

Scorch often appears on the side suddenly exposed to stronger light. Tissue becomes pale or yellow and forms a permanent scar after dying. A temporal link to moving the plant into sunlight is a clue, but does not replace assessment of stem temperature and hydration. Acclimatisation must address the new exposure. Abruptly turning a plant that has long remained stationary may expose its previously shaded side. Fertiliser or fungicide will not “heal” damaged epidermis. Healthy new growth indicates whether improved conditions are helping. [Fleischer and Schütz 1978, pp. 183–185] [Seneta b.d., pp. 105–108]

Growth under insufficient light may be elongated, narrower and less strongly coloured, with different spine development. However, stem length alone is not diagnostic. The species' normal growth form must be known, and new sections compared with earlier ones. Increasing the fertiliser dose to “strengthen” an elongated cactus does not remove the light limitation. Lighting is improved gradually, taking growth activity and temperature into account. [Fleischer and Schütz 1978, pp. 183–185]

Seedlings need not respond like adult plants. In an experiment with Opuntia streptacantha, a combination of shade and watering favoured the development of young plants. Changes in chloroplast distribution were also found under strong light and drought. The study supports distinguishing the requirements of different developmental stages, rather than a simple rule that “every cactus needs maximum sun”. [Delgado-Sánchez et al. 2013]

Roots in wet substrate and actual dehydration

In substrate that remains water-filled for a long time, the proportion of air-containing spaces decreases. Oxygen deficiency limits root function, so a plant may wilt despite water being available in the pot. The moisture of the entire root ball, the drainage hole and living roots are assessed first. Adding water to a wet root ball does not resolve the loss of root function. Once the problem is identified, conditions in the root zone are improved and checks made for developing rot. [Rutgers New Jersey Agricultural Experiment Station b.d.] [6-WWW] [Fleischer and Schütz 1978, pp. 183–188]

Stem shrinkage during a dry period may be reversible use of stored water, but severe, prolonged wilting requires checking the roots and looking for pests. No single watering calendar is established for all species. Substrate moisture, season, temperature and root system condition explain the symptom better than the number of days since the last watering alone. [Fleischer and Schütz 1978, pp. 183–185] [Hinz and Abramowicz 1986, pp. 102–105]

Chilling, frost and thermal shock

Chilling injury does not require tissues to freeze. Warmth-loving plants may tolerate above-zero temperatures poorly. After freezing, tissues may become watery and limp, then necrotic. Observations after a cold night must take account of the plant's own temperature, exposure duration and hardening. Microorganisms may subsequently colonise the disintegrating tissues. A detailed distinction between tolerance, acclimation and recovery is presented in the chapter on the physiology of frost tolerance. [Fleischer and Schütz 1978, pp. 127–129, 183–185] [Hinz and Abramowicz 1986, pp. 100–102]

Deficiency, salinity and misinterpretation of colour

Chlorosis may accompany a nutrient deficiency, an unsuitable pH that hinders uptake, or poorly functioning roots. Water, substrate and roots are assessed before fertilisation is adjusted. Increasing fertiliser concentration when salinity is already high worsens the osmotic problem. Electrical conductivity, EC, indicates the total ion concentration, but does not by itself identify which element is deficient. [7-WWW] [Fleischer and Schütz 1978, pp. 183–185]

A colour change is not a simple indicator of one nutrient or a diagnosis of a virus. An older description of reddening attributed the colour to anthocyanins. In cacti, red-purple pigmentation can be produced by betacyanins, which belong to the betalains. Mosco's anatomical study demonstrated their presence in the hypodermis and outer layers of chlorenchyma. [Mosco 2012] Ageing epidermis at the base and a permanent scar are distinguished from actively expanding necrosis. The rate and location of changes are part of the diagnosis. [Seneta b.d., pp. 105–108] [Hinz and Abramowicz 1986, pp. 28–32]

Differential diagnosis table

The table organises inspection, but does not assign any one symptom exclusively to one cause. Persistent or progressive changes require reassessing the diagnosis. [Fleischer and Schütz 1978, pp. 183–188] [9-WWW]