Plant Diseases and Their Causes: Notes for Exams

A plant disease is any lasting disturbance of a plant's normal working caused by a living agent or by a shortage of a nutrient. The subject is worth marks in every general science paper because the questions are pairings: a disease, a crop and a causal organism. Nearly all of them come from five groups of causes. Fungi are the largest group and account for most crop losses. Bacteria cause the rots, blights and cankers. Viruses cause the mosaics and leaf curls and are carried by insects. Phytoplasmas, once called mycoplasma-like organisms, cause the yellows and the little-leaf group. Nematodes attack roots. Alongside these living agents are the physiological or deficiency diseases, in which nothing infects the plant at all and a missing mineral produces the symptom.

Diseases caused by fungi

Fungal plant pathogens produce spores that travel on wind, water, seed and soil, and they thrive in humid weather, which is why most Indian fungal epidemics follow the monsoon. The single most famous example in the history of science is late blight of potato, caused by Phytophthora infestans; the epidemic of the 1840s destroyed the potato crop of Ireland and caused the famine and emigration that followed. The study of that outbreak made Anton de Bary the founder of plant pathology, because he proved that the fungus was the cause of the disease and not a result of it.

Diseases caused by bacteria

Bacteria enter a plant through wounds, insect punctures and natural openings such as stomata, and they spread in splashing rain and on tools. Citrus canker, which raises corky brown spots on leaves and fruit, is caused by a species of Xanthomonas, and so is bacterial blight of rice, one of the most damaging diseases of the crop in India. Bacterial wilt or brown rot of potato is caused by Ralstonia solanacearum, and crown gall, in which the plant grows a tumour at the collar, is caused by Agrobacterium tumefaciens, the same bacterium that biotechnology now uses to carry genes into plant cells. Tundu disease of wheat is unusual because a bacterium and a nematode act together.

Diseases caused by viruses and phytoplasmas

Plant viruses do not move on their own: an insect, a nematode or infected planting material must carry them. Tobacco mosaic disease matters twice over, because the tobacco mosaic virus was the first virus of any kind to be discovered and later the first to be crystallised, so the whole science of virology begins in a plant disease. Leaf curl of cotton, chilli and tomato and yellow vein mosaic of okra are carried by the whitefly; many mosaics of pulses and cucurbits are carried by aphids; bunchy top of banana, which shortens the plant and bunches its leaves at the top, is also viral. Phytoplasmas, which are bacteria without cell walls, are spread by leafhoppers and cause little leaf of brinjal, grassy shoot of sugarcane and spike disease of sandalwood.

Diseases caused by nematodes

Nematodes are microscopic roundworms of the soil. Root-knot nematodes of the genus Meloidogyne make galls on the roots of vegetables, so the plant wilts in the afternoon even in moist soil, and the molya disease of wheat and barley, important in the dry north-west, is caused by a cyst nematode. Because the damage is below ground, nematode disease is usually diagnosed by pulling up a plant and looking at the roots.

Deficiency or physiological diseases

Here no organism is involved. A plant short of a mineral shows a symptom that looks like a disease, and the cure is the missing nutrient, not a spray against a pathogen. These are favourite examination questions because each symptom has a single mineral attached to it.

How plant diseases are controlled

Four measures cover almost every answer. Healthy, treated seed and certified planting material keep a pathogen out of the field in the first place. Crop rotation starves a soil-borne fungus or nematode of its host. Resistant varieties bred for the purpose are the cheapest long-term answer, and the dwarf wheats of the Green Revolution were selected for rust resistance as well as for yield. Chemical control uses fungicides and bactericides, of which the oldest is the Bordeaux mixture, a suspension of copper sulphate and lime devised in France against downy mildew of the grapevine and still sprayed in orchards. For virus diseases there is no cure once a plant is infected, so control works on the vector: destroy the whiteflies and aphids, remove infected plants, and plant virus-free material.