Leaves first show a burning or scorching around margins, usually with older leaves being affected first. The marginal scorch progresses inward until the entire leaf withers and drops. The extent and rapidity of leaf drop is dependent on the type of plant and cause of damage.
Many types of plants show symptoms of withering and scorching when unfavorable conditions exist. These conditions can be grouped into the following categories:
- Root Failure – One of the most common causes of withering and scorching. Roots may be damaged by such things as diseases, insects or factors that exclude oxygen from the soil. Certain insects, diseases or nematodes may devitalize enough of the root system to seriously hamper its function. When this occurs the leaves are not furnished with enough water and nutrients to supply to top. Such is the case with plants infested with root knot nematodes. Knots or galls form on roots and the entire system is devitalized to the point that the plant may be clorotic and stunted along with scored leaves.
Root failure often results when oxygen is not available to roots. Such is the case with overwatering, fill soil, soil compaction or natural gas leaks. Roots are killed when oxygen is excluded, carbon dioxide accumulates and the pH of the soil is reduced. Gases and toxic compounds may also be produced by anaerobic bacteria under these conditions and some can survive much better than others. Post oak trees are more sensitive than live oak, for example.
Many things can damage roots and careful observation is necessary to properly identify causes. Withering and scorching caused by root failure results from roots being unable to absorb and translocate enough water and nutrients to adequately furnish the plants.
- Soil Deficiencies or Toxicities – Another common cause of withering and scorching is shortage of water and nutrients. If these are short enough in supply the entire leaf area cannot be furnished with water. When this condition exists, areas furthest from the source (leaf margins) will begin to scorch. As the problem becomes more acute, scorching will progress toward the center of the leaf. Watering according to need with good quality water and fertilizing according to soil test represent the best means of prevention.
Toxicity can also result in withering and scorching of foliage. Toxic materials vary in the way they affect plants. Excessive use of fertilizer burns roots and causes root failure described earlier. Certain herbicides may kill roots directly while others translocate to foliage tissue, with these often accumulating on leaf margins and scorching occurs. Many substances are toxic to plants and their modes of action differ.
- Atmospheric toxicity – Certain air pollutants may produce a scorching of the leaf surface. Several described pollutants are capable of causing damage and these are identified in the section on air pollution. Sulfur dioxide causes a scorching around leaf edges initially. This could be confused with damage caused by root failure or toxic substances.
- Vascular diseases – Plants or trees infected with vascular disease organisms such as Fusarium wilt, Verticillium wilt, or oak wilt may have scorched leaves. Water transporting vessels become plugged and prevent normal translocation.