Diseases, Pests and Weeds

A plant diseases are caused by many organisms, such as bacteria, virus, fungus and insects. Leaf and stem diseases are the result of bacteria, virus and fungal infections; whereas soil-borne diseases affecting the roots are transmitted by various fungi and nematodes. The plants also get sick due to environmental factors, such as air, water, pollution, deficiency or excess of nutrients or sunlight and the wrong kind of climate.

Fungal diseases are quite common and are encouraged by atmospheric humidity, dew and reduced light conditions, such as on overcast days. Fungus spreads over the entire plant, damaging its appearance and maturity. Common among fungal infections is powdery mildew, which appears as a white or grey powdery coating on leaves, tender stems or flower buds. These turn reddish afterwards.

Another kind of fungus is rust, which causes the leaves and stems to be covered with rust colored spots. Then, the leaves fall off. Sometimes, rose plants are affected by fungal black spots, causing the leaves to fall.

Viruses invade the plant tissue, reproduce in it and cause abnormality in growth, variegation of foliage, color distortion of the blossoms and curling of the leaves (leaf-curl virus). In some diseases, like rose mosaic, the foliage becomes variegated but the plant is otherwise unharmed. In some tulips, the virus causes bizarre, striped flowers.

There are also soil-borne diseases, which spread by infecting the plant roots until the roots are severely damaged. The stem collapses and the plant or seedling topples. Water mould fungus is a type of soil, borne disease, which is caused by a fungus that thrives in excess water, especially if there is less air in the soil. The first symptom of this disease is the dropping off of leaves, as the roots decay and the plant finally dies. On the other hand, under,watering causes the plants to have smaller, dark-colored leaves, and the early symptoms are wilting of leaves and dropping of petals. If dry conditions persist, the plant will die, except in the case of cacti and succulents.

Insects often severely damage plants. Sometimes a magnifying glass is needed to spot them. Especially aphid larvae, which look like tiny golden or black drops to the naked eye, and are found on distorted looking tender leaves. Caterpillars are conspicuous enough and can easily be identified by their characteristic style of eating leaves. Mites are usually found under the leaves that develop yellow blotches. Like aphids, they suck plant juices. Slugs and snails bite off plant tissue, generally at night. Even ants harm plants as they carry aphids from one plant to another. Aphids and mites are the more damaging pests, because aphids spread virus diseases. The others are not as harmful. Earthworms in fact, are beneficial to plants as they aerate the soil by digging tunnels, and their casts (excreta) are rich in nutrients.

For the home gardener, the only insecticide recommended is malathion, as the toxicity dissipates after several days. Dilute two millilitres of malathion in one litre of water, and spray this solution over the plants. Bavastin and Diathane M45 are two strong fungicides, which are available at any garden store. These should be used with extreme caution, and only if necessary. Another is the home-made tobacco insecticide, which is prepared by boiling 100 grams of cheap tobacco and 15 grams soap in a quarter litre of water. Strain and dilute with another quarter litre of water. Spray the solution over the insects. It works, and is not toxic to humans unless you drink a lot of it.

Weeds almost always come in between the desired plants and must be pulled out, root and all, by loosening the soil around them. If left there, they compete with the plants for food and light.

Remember: Never delay weeding and allow the formation of their seeds, as seeds which fall on the ground will form new weeds under favourable conditions. The rainy season, especially, brings up many weeds that should be pulled out before the end of the monsoon.