Plant Stress and Disease

We have had a very stressful year for plants. As we go into the winter season, I thought it would be good to review how stresses affect plants and how this relates to plant diseases. The following is an article on the subject.

What are the most common environmental factors that predispose plants to disease? The list includes drought, excess moisture, temperature extremes, nutrient imbalance, wounding, and chemicals such as weed killers and pollutants.

Temperature.

Plants grow best from 1 to 40 C (optimum at 15 to 30 C). This optimum varies with plant species, stage of growth and plant part. It is very common for plants to be exposed to temperatures outside their normal range. Low temperature stress (freezing or chilling) interferes with cellular processes and can cause frost damage. Fluctuation in temperature can impact plant acclimation to cold temperatures. High temperature stress, which most often occurs in an artificial environment, affects the action of enzymes and can denature proteins.

Moisture.

Both insufficient moisture and excess moisture can harm plants. Drought stress occurs when water loss from the leaves exceeds water uptake in the roots. This stress is the most damaging to plants and is responsible for more plant troubles than any other environmental factor. Drought can do both short-term and long-term damage to plants by reducing photosynthesis, shrinking tissues, impacting water transport (which results in wilt and scorch), and impairing root function. In the longterm, plants stressed by drought are predisposed to attack by many organisms that cause plant disease. Drought stress combined with temperature stress can cause summer drying or winter burn. When excessive soil moisture occurs in the root zone, oxygen is reduced in soil pores. Fibrous roots die-off, which leads to symptoms of water stress. Such roots are invaded by pathogens in soil. In addition, populations of anaerobic organisms increase in soil, which leads to a buildup of toxins (organic acids, methane, and alcohols). Denitrifying bacteria convert nitrates in soil to N2, causing nitrogen deficiency in plants.

Injury

Wounding, mechanical injury, and human activities can impact plant growth and predispose plants to disease. Many opportunistic organisms, such as those that cause cankers in plants, require wounds to invade plant tissues. Human activities, such as poor planting practices as well as improper placement are all predisposing factors for plant disease.

Nutrients.

All plants require 20 essential nutrients for growth and reproduction. C and O2 are supplied in the air, the rest are supplied in water or in soil. Macronutrients (N, P, K, Ca, Mg, S) are required in large quantities as part of plant structure or in regulatory functions. Micronutrients (B, Cl, Cu, Fe, Mn, Mo, Zn) are required in small quantities as part of plant structure or in enzyme complexes. Symptoms due to extremes in soil nutrients depend on the functions of a particular element within a plant. Extremes in nitrogen predispose plants to many diseases.

Chemicals.

These include pollutants (ozone, sulfur dioxide, nitrogen oxide, fluoride, or ethylene), herbicides, salts, and other agricultural or household compounds.

Extracted from "The Relationship of Plant Stress to Plant Disease" by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers Univerisity in the May 31, 2007 edition of the Plant and Pest Advisory; Landscape, Nursery, and Turf edition.