Bacteria, fungi, and viruses all can cause plant diseases. And all these can stem from several factors, like nutritional deficiencies, air pollution, unfavorable environment and soil, and too little/too much sunlight. Bacteria are single celled parasites that need to obtain their food from their host plants. Fungi are multi cell parasites that do the same thing as bacteria, except on a much larger scale, and viruses are very small pathogens that can infect everything from bacteria to plants.
Of these, fungus infestations are the commonest plant pathogens, and one of the commonest fungi is leaf spot. These leaf spot fungi manifest in a variety of colored lesions like black, brown, red, or yellow. After developing the spots, the affected leaf will drop from the plant. Bad cases can cause mass defoliation and severely affect the tree. This problem is caused by a fungus that spreads from one infected plant to another. The spores (which are airborne or waterborne) are also spread through dead leaves and other plant debris.
There are 3 common types of leaf spot diseases:
Anthracnose fungi (Colletotrichum graminicola) attack young leaves and shoots in spring; occasionally older leaves as well. The fungus causes the appearance of large, dark blights on leaves, and cankers on twigs and small branches. Wet weather encourages the growth of this fungus, while it diminishes in dry weather. There are other types of anthracnose fungi that cause large cankers and sometimes can be fatal to the tree.
Certain anthracnose cases are difficult to eradicate. There is not much you can do at control, apart from pruning and spraying the plants with fungicides like chlorothalonil and polyoxin. For best results, spray the fungicides during the wet phase of spring, when the leaves unfold, 2-3 times a fortnight. Destroying the infected plant is a last resort, but may be necessary in bad cases.
Black spot (Diplocarpon rosae) is a fungus mostly affecting roses, and causes circular black spots which spread from lower leaves up to the top. The circles are very small, but appear as a big blotch in heavy infestations. The fungus is spread by its spores which immerse in water and then latches onto newly emerging leaves due to splashing. Remove all old leaves, dead leaves, and if necessary, spray the infected areas with fungicides (chlorothalonil, folpet).
There are several kinds of scabs, which affect different types of trees and plants. One of them (Cladosporium carpophilum) affects the fruits, twigs, and leaves of apricots, peaches, and plums. Another, the apple scab (Venturia inaequalis) affects apples and may cause heavy fruit loss if not tackled early on. Still another scab is the pear scab (Fusicladium pyrorum). Scab is common in wet weather, and thus, the season for scabs is early spring.
Unlike other leaf infesting fungi, the dark lesions of scab are the actual fungus growth on the affected area and not areas of dead tissue. While it does not kill its host tree, scab is still a troublesome problem.
Remove all the leaf litter, and prune out the affected areas as much as possible. Traditional fungicides like captan, mancozeb, or wettable sulfur have long been used to control scab. Spray just before flower buds open, spray again after they show color, and one more time when two thirds of the petals have fallen.
Other control measures
Fungicide is of course, the last resort in controlling leaf spot problems. Organic fungicides can be used, but are usually those of the copper based type which have limited effectiveness (although they do contain the problem from spreading). Do try to ask at your nursery for scab resistant cultivars when you purchase seedlings.
Also, choose your planting location well. Try not to plant in a low lying or shaded area because of the moister microclimate and poorer water circulation in the soil, and clear up fallen leaves and fruit to minimize the available habitat for the fungi spores to colonize. Unless the leaf spot problem is bad, in many cases this is a problem that many gardeners have to put up with (and keep in check).
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