PLANT DISEASES

A large number of diseases occur on various crops. These diseases may be caused by fungi, bacteria, viruses and other pathogensStem rusts, white rusts, smuts, blights, wilts downy mildews, powdery mildews, cankers etc. are

of the important diseases. Symptoms, causal organisms, etiology and control measures of some of the more common diseases are discussed in this chapter.

DISEASES CAUSED BY FUNGI

1. LATE BLIGHT OF POTATO

Late blight of potato was initially an endemic disease of South America. Because of quick transportation this disease spread to U.S.A. and Europe in 1830-1840. This disease caused famous Irish Famine in 1845-1846 where it caused havoc in potato fields and completely destroyed the potato crop andresulted in the death and migration of millions of people. In India the disease first appeared in Nilgiri hills between 1870-1880. Then it was reported from Darjelling. Because of high temperature in the plains, the disease did not appear in the plains of India. It appeared for the first time in the plains of Hooghly district in Bengal in 1899-1900. Since 1943, this disease has been making regular appearance in the plains of North India due to low temperature provided by the cold storage facilities of potato tubers.

Causal Organism. Phytophthora infestans, a fungus belonging to class Oomycets. Symptoms. First

symptoms of the disease appear as hydrotic spots at the margins and tips of lower leaves (Fig. 30.1). Thesespots then become necrotic and turn brown to black coloured. The necrotic areas are sometimes bordered with chloranemic borders. In moist weather, these spots spread and cover the whole top of the plant. The host plant decays and produces a foul smell. The diseased tubers are somewhat deepressed and wrinkled. The upper skin of tubers becomes

2. Later on, rusty black spots develop below their skin. The tubers decay before the harvest and turn pulpy.The disease develops under wet conditions, high relative humidity, cloudiness, moderate rainfall and temperature not

not exceeding 22–23°C. The disease is transmitted under damp, cool and windy weather.

The mycelium grows in the intercellular spaces of host tissues. The mycelium produces haustoria which penetrate host cells ar absorb nourishment.

Disease Cycle (Fig. 30.2). Infection occurs through the tubers, used as seed whichused in cold storage. The fungus multiplies asexually through conidia sporangia. The infection spreads by means of sporangia which are lemon-shaped in outline. Disposal of sporangia occurs by wind and germinate on the leaves of healthy plants. Sporangia germinate to produce biflagellate zoospores. Under favourable conditions the zoospores infect

host plants. The zoospores encyst and infect the plants by producing germ tubes. The germ tube directly penetrates through epidermal cells or enters the host leaf through stomatal openings.

Potato tubers are also infectd while in the field and still attached to the plant or they get the infection during harvest and sometimes during storage. Early infection is also responsible for reduction of the size and number of tubers.

Control Measures 1. Disease free tubers should be used for

cultivation. 2. Debris of crop should be removed from

the field. 3. Disease can be controlled by spraying

Dithane M-45, Brestan and Difolatan 80 WP fungicides. The spraying should be started much earlier to the appearance of the disease. The first spray should be carried out when the plant is one month old. Bordeux mixture is the most effective spray fungicide. The strength of early spray should be 4:4: 50 while for later spray the strength

should be 6:6: 50. 4. The tubers should be dipped for about

one and a half hour in 1:1000 mercuric

chloride solution before storage. 5. Healthy tubers should be selected

before storage. 6. Diseased plants should be destroyed

and burnt before harvesting. 7. Disease resistant varieties should be

used. Attempts have been made to breed resistant varieties but it is not a successful practice because of the occurence of many physiological races of the fungus in India.2. LOOSE SMUT OF WHEAT

Loose smut of wheat is a serious disease and

causes heavy loss when susceptible varieties with infected seeds are grown year after year. Disease is called smut due to black coloured spores formed within the host tissues and gives burnt or charred or sooty appearance to the infected part. These can be grown on artificial media, so, are not obligate parasites.

Causal Organism. Ustilago tritici is a causal organism of loose smut of wheat.

Symptoms. Smut fungi bring about severe loss due to destructive nature of disease and every head of the affected plant may be converted into a black mass of spores and no grains are formed. The disease first becomes evident when the blackened ears emerge from the leaf sheaths (Fig. 30.3). The diseased ear emerges out of boot leaf a little earlier than healthy ears'. The diseased

ear contains powdery mass of smut spores enclosed within a thin silvery membrane in the beginning but get exposed due to its rupture before the emergence of the ear.

Cool and humid weather, at the time of flowering, is favourable for the development of disease.

Relationship with the Host. Dikaryotic mycelium is present throughout the plant which grows in the intercellular spaces. It sends intracellular haustoria to absorb the nourishment. The sporulation takes place in the floral parts including ovary walls except the rachis.

Disease Cycle (Fig. 30.4). This is a seed-borne disease. Spores (Chlamydospores) are dispersed by wind or rain leaving behind the naked inflorescence axis (rachis). The spores germinate on the stigma of healthy flowers. They germinate to produce sporidia. The sporidia fuse with each other and dikaryotisation occurs. The dikaryotic hyphae penetrate into the ovary.

It grows alongwith the development of the s

seed. The fungus survives in the seeds in the form of dormant mycelium, which becomes active as soon as the infected grain is sown in the field.