Healthy leaves are typically green and firm, but occasionally leaves will begin to show signs of disease or affliction by turning yellow, brown or developing spots or blight that make the surface appear mottled. Most leaf spot disease appears in the early spring and can be treated to protect the tree's overall health.
Look for leaves that curl and begin to show brown or black spots. The stems may also turn dark. These signs of leaf and shoot blight are most common in aspen trees. A curled leaf appearing on a stone fruit tree indicates fungus that must be eradicated with several well-timed applications of fungicide. Apply when leaves first appear in spring and repeat every two weeks. Keep leaves dry by watering in the morning.
Inspect leaves for small spots that are yellow to orange in colour. These spots bear powdery spores on leaves, often seen in late summer. Leaves affected with a similar fungus, powdery mildew, are covered with a fine white powder and curl in the later stages. Fungus can best be controlled through prevention and good housekeeping in the garden. Remove all infected leaves and isolate affected plants, if possible. Prune to allow for greater air circulation and spray with fungicide as directed.
Watch for black or brown specks that appear irregularly on the leaf. Over time, these will grow and appear blotchy. This fungus, marssonia leaf spot, is most common in late summer. The leaves appear as if they have sustained frost damage in the absence of frost. Keep leaves and the surrounding areas as dry as possible and apply a fungicide prior to bud break. Repeat two weeks later.
Inspect tomato plants for wilting, yellowing leaves and large, blackened nodes. According to Cornell University, this indicates pith necrosis, a bacterial infection. Avoid excess nitrogen and check to make sure the soil is well-draining, with good air movement. Keep humidity low and consider raised beds. Select seeds that have been treated with hot water or bleach to control bacterial and fungal diseases.
Identify a mottled yellow colour on leaves that crosses over the veins, which is caused by bacteria spread by the insect psyllids, or jumping plant lice. Use an insecticide spray once every two weeks and consider eliminating infected trees to control the spread of the bacterial infection.
Examine the top of the tree and observe the canopy as a whole. If it looks thin with pale, wilting leaves, the tree most likely has root and crown disease, which is often caused by poor drainage and too much water. Prevent the disease by planting trees on an uphill slope so that water can drain properly. Avoid overwatering and spray trees with a phosphonate product in the fall and the spring.
Prevent leaf disease by raking the garden area frequently in fall. Prune diseased sections of trees well to avoid spreading. Plant trees at sufficient intervals to allow for good air circulation. Apply fungicide when it works best: prior to bud break and before potential problems emerge. Once they have already occurred, leaf problems are far harder to cure. Prevention is paramount.