When a living organism's cells or tissues die or degenerate, the condition is called necrosis. In a plant, necrosis causes leaves, stems and other parts to darken and wilt. Necrosis weakens the plant and makes it more susceptible to other diseases and pests. Some plant diseases caused by a specific agent have the word necrosis in their name, such as tomato pith necrosis. However, necrosis is typically considered a symptom rather than a disease itself.
Plants don't require large amounts of nutrients, but necrosis can occur when nutrients aren't available. Depleted soil, incorrect soil pH for the plant or the wrong fertiliser can cause nutrient deficiency. Extremely low phosphorus will initially cause the plant's leaves to look very green, but eventually the older leaves turn necrotic. Other nutrient deficiencies that lead to necrosis include potassium, nitrogen, boron, iron and nickel.
Viruses require the cells of plants to grow and reproduce. As the viruses use the plant as a host, the plant's energy is diverted to the virus rather than to the plant's growth. Over time, this energy depletion leads to necrosis in the area where the virus lives. Viruses typically only infect specific areas of a plant and rarely kills the entire plant. Remove and destroy the affected area of the plant. If the problem persists, destroy the entire plant and replace it with a virus-resistant variety.
Fungal & Bacterial Problems
Fungal and bacterial problems in plants often present on the leaves. The infection starts as grey, white, red, yellow or brown spots on the leaves. If left untreated, the infection will eventually turn the leaf necrotic. Fungi typically invade from the plant surface, while bacteria enter through openings on the plant. In addition to selecting disease-resistant plants, you can avoid bacterial and fungal diseases by not letting plants stay wet, maintaining air flow around plants and removing affected leaves as soon as possible.
Although some nematodes benefit the garden by eating pests, others infect plant tissue and cause necrosis. Aphelenchoides sp. nematodes eat their way through the spaces in between leaf veins, eventually causing structural problems and necrosis. Applying beneficial nematodes, using nematode-resistant plants and rotating crops, helps prevent nematode problems.