eChapters

COTTON (Gossypium spp)
Fungal diseases
1) Damping off
Symptoms: The seeds may rot. Initially the roots and hypocotyls become water soaked and pale green in colour. The hypocotyls become constricted above the soil level and seedlings fall down. Causal organism: Pythium aphanidermatum
Management
i) Seed treatment
ii) Growing of resistant varieties.
2) Root rot
Symptoms: Roots of infected plants rot and show shredding of the bark. The affected plants can be easily pulled out.
Causal organism: Rhizoctonia solani

2.1 Loose Smut of Wheat

The disease is prevalent in all the wheat growing areas in the world. However, it is a serious problem in humid and sub-humid areas. The incidence of the disease is comparatively more in cooler and humid parts of northern India as compared to the drier and tropical parts of the country. Being a simple interest disease epidemiologically, this is a threat to the cultivation of wheat for seeds. The disease is therefore, dreaded by seed growing agencies who have to treat the seeds with costly chemicals.

CEREALS

Rice (Oryza sativa)

1. Rice blast
Pyricularia oryzae

‘Rice blast’ occurs in more than 80 countries across the world. The disease is more prevalent in Thailand, Japan, America, Guyana, Philippines and India. It is particularly serious in places, where rice is irrigated or receives heavy rainfall and where high levels of nitrogenous fertilizers are applied. Rice blast was first reported in China in 1637, in Japan in 1704 and in India in 1913. In India, it is found in all rice growing states, particularly in Telungu Desam, Karnataka, Maharashtra, Uttar Pradesh. Assam, Tripura, West Bengal, Bihar, Orissa, Himachal Pradesh, Punjab, Madhya Pradesh, Gujarat, Kerala, Tamil Nadu and Pondicherry and causes considerable damage to rice crop. In Tamil Nadu, the disease occurs in all rice growing districts and attains epidemic proportions in some seasons. Several rice blast epidemics have occurred in different parts of the world, causing yield losses ranging from 50 to 90 %. In Tamil Nadu, a serious epidemic of the disease occurred in 1919 in the Thanjavur Delta.

Symptoms. The disease, though chiefly a foliage disease, attacks other parts of rice plants, such as leaf sheath, rachis, joints of the culms and even the glumes. On the leaves, the initial symptoms appear as small bluish or grayish specks, which remain circular in older leaves. On young leaves, they enlarge and become eye-shaped, diamond-shaped or spindle-shaped spots, extending over several centimeters in length and about 1.0 cm. in width. The central portion of the lesion appears pale green or grayish green and water-soaked in the beginning. In older spots, the center becomes white to gray with a dark-brown margin surrounded by a yellow hallow. The lesions may coalesce, cover large area of the leaves and eventually the entire leaves are killed. The shape, size and color of the spots vary depending upon the variety, growth stage of the plants and environmental conditions. Similar spots may be formed on the leaf sheath also. During severe incidence, large areas of the leaves dry and die and the entire crop presents a burnt appearance. Hence the disease is known as ‘blast disease’. Leaf blast is more severe during the seedling and maximum tillering stages. In the nursery, the seedlings show numerous spots on the leaves, which wither and die. In case of severe infection, large number of seedlings may die. Blast also affects the leaf collar, which results in the death of the entire leaf.

The disease attacks the stem-nodes at the heading stage leading to production of ‘white panicles’ or the stem may break at the infected node. Rarely the internodes are affected. Further, at heading the fungus also attacks the panicle neck, which is girdled and becomes shining black and shrivelled. This is the most destructive phase of the disease and is known as ‘neck blast’, ‘neck rot’ or ‘panicle blast’. If infection of the panicle neck occurs early, the grains become chaffy and the panicle remains erect. If panicle neck infection occurs late, the grains become partially filled and because of the weight of the grains, the panicle breaks at the neck- node and droops. Sometimes, only parts of the panicle and joints of the culms and some glumes are infected and develop brown to black spots (Fig. 1).

Under conditions highly favorable for the pathogen, even the varieties resistant to the fungus under normal conditions are also attacked and small, round or short elliptical spots develop. However, the spots normally do not enlarge as typical blast spots.

BERSEEM CLOVER (Trifolium alexandrinum)
Fungal diseases
i) Damping-off
Symptoms
Pre-emergence damping-off: Seeds decay inside the soil.
Post-emergence damping-off: Sometimes after germination water-soaked blighted seedlings develop. Seedlings collapse and wilt. Stem and root tissues rot below the soil surface.
Causal organism
i) Pythium spinosum var. spinosum
ii) Rhizoctonia solani
Favourable condition
i) Promoted by high humidity
ii) Poor air circulation.

3.1 Damping-off Disease of Seedlings

The disease is quite common in nurseries or seedbeds all over the world. It is virtually a serious threat to almost all the nursery crops of vegetables and other crops including tomato, chillies, cauliflower, cabbage, brinjal, cucurbits, tobacco and a number of other crops. The economic importance of the disease is more because it takes the toll of precious seedlings in the very initial stages of the crop cultivation. The seed can become infected with a fungus which may cause darkening and softening of the seed leading to its decay. Similarly, seedlings may also be infected while sprouting or even after emergence. The disease is more serious in heavy soils with poor drainage. Abundant soil moisture with 90-100% humidity and temperature around 200C is favourable for the disease.

Diagnostic Symptoms

• At first pale-green chlorosis, typically appear on lower leaves and extending up the plant.Leaves eventually take on a dull-yellow colour, wilt and the plant collapses and dies.

Downy Mildew

Plasmopara viticola

• Downy mildew of grapes was catastrophic in France in 1878.

Anthracnose

Colletotrichum lindemuthianum

(Sexual stage: Glomerella lindemuthianum)

Diagnostic Symptoms

• The pathogen can attack all aerial parts at any stage of plan growth.
• Main symptoms are circular, black, sunken spots with dark center and bright red orange margins on leaves and pods.
• In severe infections the affected parts wither off.
• Seedling get destroyed due to infection soon after seed germination.

Tikka or Leaf Spot

Cercospora arachidicola/ Phaeoisariopsis personatum

Economic Importance

• Tikka disease of ground nut is regarded as the most important fungal disease of this crop.
• The infection appears when the plants are one to two month old; one can see this disease in the month of September in the plains of our country.
• Losses due to this disease may go up to 15-50%.

FRUIT CROPS

Banana (Musa species)

1. Panama disease of banana
Fusarium oxysporum f.sp. cubense

‘Panama disease’, ‘Fusarium wilt’ or ’vascular wilt’ of banana is one of the most serious and destructive diseases of the crop. The disease was first reported from Australia in 1874. Now, it is widely distributed all over the world, including America, Africa, Sri Lanka, Burma, Thailand, Malaysia, Indonesia, Hawai, Fiji, the Philippines, India, Australia and New Zealand. In India, it is prevalent in the states of Assam, Bihar, Karnataka, Kerala, Tamil Nadu, Telungu Desam, Maharashtra and West Bengal.

Symptoms. Usually the symptoms are manifested, when the banana plants are about 5 months old. However, under highly favorable conditions for the pathogen, even 2 - 3 months old plants show wilt symptoms. Initial visible symptoms appear as pale yellow streaks, lengthwise in the petioles of the oldest, lowermost leaves. Two types of symptoms follow this. In the first type, the old leaves turn yellow progressively and finally they break from the petiole and hang down. In the second type, the leaves break from the petiole and hang down, without becoming chlorotic. Gradually, all the leaves collapse and hang down, except the crown, which alone stands upright. The newly formed leaves show yellow blotches and wrinkling of the lamina. Often, the leaf sheaths covering the pseudostem show longitudinal splitting above the ground level. But, sometimes this symptom may not appear. The affected plants may die completely in about 4-6 weeks after the appearance of yellow streaks on the petioles.

The vascular strands show discoloration varying from yellow to dark- brown, which is the characteristic internal symptom of the disease. The discoloration usually appears first in the outer leaf sheaths and extends to the vascular strands of the pseudostem. Vascular discoloration is more pronounced in the corms, but is not common in the roots. When the rhizome of an infected plant is cut transversely, the discoloration of the vascular bundles is distinctly seen as numerous, pin-point like dots all over the cut surface. The roots of affected plants become black and rotten. The suckers growing from the diseased corms are systemically affected and they wilt and die very soon (Fig. 71).

The causal organism. The mycelium of the fungus is hyaline, septate, much branched and mostly intracellular. Few hyphae may also be present in the intercellular spaces. The hyphae are largely confined to the vascular bundles, often filling the cavity of the vessels. The fungus produces three types of spores viz., macroconidia, microconidia and chlamydospores. The sporodochia bearing conidiophores and conidia emerge through the stomata on the petioles and leaves. The conidiophores are vertically branched. Macroconidia are borne at the apical ends of the main and lateral branches and are formed in succession one after another. They are thin-walled, hyaline, pedicellate, sickle-shaped, with both ends somewhat pointed, 2 - 5 septate, mostly 3-septate and measure 22.0 - 36.0 x 4.0 - 5.0µ in size. Microconidia are produced from the branches of conidiophores or from the tips of hyphae in very large numbers. They are single-celled or two-celled, thin-walled, hyaline, ovate or elongated and measure 5.0 - 7.0 x 2.5 - 3.0µ in size. Terminal or intercalary chlamydospores are formed from the hyphae, as well as from the conidial cells. They are oval or spherical, thick-walled and usually occur in pairs. They measure 7.0 - 13.0 x 7.0 - 8.0µ in size (Fig. 71).

The fungus is a facultative parasite. It can invade the host mainly through wounds in the corms or roots. After entry, the fungus develops extensively in the vascular tissue of the corm and then proceeds systemically to the leaf sheaths and pseudostem through the vascular system. The fungus colonizes the vascular bundles and produces masses of mycelium, which bear conidia and chlamydospores, filling the cavity of the vessels. The plugging of vascular elements by the fungus obstructs the translocation of water and nutrients to the aboveground parts of the plant leading to wilting and death of affected plants.

Mode of survival, spread and epidemiology. The fungus is soil-borne in nature. It can survive saprophytically in the diseased corms and other plant parts for prolonged periods and cause fresh infection. The chlamydospores can also remain in a viable state in the soil and diseased plant parts for a long time. The disease can be easily spread through spores present in the infected banana trash and soil, which may be carried in surface flooded water. Suckers used for planting from diseased areas to other places spread the disease easily. Contact of the roots of healthy plants with the roots of diseased plants carrying the spores also leads to fresh infection. The entry of the pathogen is mostly through wounds in the roots or corms, especially deep wounds up to the xylem vessels, caused incidentally during cultural operations or by soil-inhabiting pests, particularly nematodes.

Light textured, sandy loam soils, with acid reaction and low soil moisture of 25 % favor the occurrence and development of the disease. Ratooning of infected banana plants increases the disease incidence to a great extent and leads to continuous accumulation of soil inoculum.

Wilt

Fusarium oxysporum f. sp. lentis

Economic Importance

• The disease is present in countries such as India, Jordan, Nepal, Pakistan, Syria, Turkey, Russia, France, Hungary, Ukraine, Egypt, Ethiopia, Morocco, Sudan, Tunisia, Canada, Argentina, Chile and Uruguay etc.
• It may cause 5-10% yield losses but sometimes severe damage may result complete crop failure under favorable conditions.

Diagnostic Symptoms

• Symptoms can occur at both the seedling and plant developmental stages and appear as patches in the field

• It usually occurs near or at the reproductive stages (flowering to podfilling) of crop growth

Wilt

Fusarium oxysporun lini

Economic Importance

• The wilt is very serious disease and occurs in almost all the countries where linseed is grown.
• Continuous cropping in the infected field makes the soil sick.
• Yield losses upto 80 percent has been reported due to this disease. However, in severe cases crop is destroyed completely.

Southern Leaf Blight

Helminthosporium maydis (Syn: H. turcicum)

Economic Importance

• The first epidemics of the Southern corn leaf blight also known as Maydis leaf blight took place in the USA in 1970.
• A highly virulent strain called Race T appeared on maize hybrids with Texas male sterile cytoplasm.
• The disease development was related to the growing of maize with T-cytoplasm on large areas.
• Since 1990 the use of cultivars with T male sterile cytoplasm has been forbidden in selection.
• Presently, the cultivars with M and C types of sterility are used.

Anthracnose

Colletotrichum gloeosporioides (Glomerella cingulata)

Diagnostic Symptoms

• Disease produces symptoms like leaf spot, blossom blight, withertip, twig blight and fruit rot

• Symptoms on leaves show as gray to brown spots with dark margins and a yellow halo. The spots later enlarge and coaleasce to form sizable necrotic areas.

Botrytis Gray Mould

Diagnostic Symptoms

• Appearance of a gray, fuzzy mold on flowers, leaves or stems of or plants. Also called, “gray mold”.

MINT (Mentha sp)
Fungal diseases
1. Stolon Rot
Symptoms: Yellowing of leaves is observed at early stage of infection. In later stage wilting of plants are noticed followed by drying of the entire plants.
Causal organism: Macrophomina phaseoli
Management
i) Stolons should be collected from healthy fields.
ii) Use of healthy and disease free stolons for planting.
ii) Treatment of stolons with Metalaxyl 8% + Mancozeb 64% WP (0.3%)
before planting.
2. Rust
Symptoms. In the spring or summer season the leaves and upper portion of stems are affected and become twisted. Yellow/ dusty orange pustules appear on the leaves and stems. In later part the pustules become black. Drying of leaves followed by defoliation occur.

FINGER MILLET (Eleusine coracana)
Fungal diseases
1) Blast
Symptoms: The above ground parts (leaf, leaf sheath, stems) of the plant exihibit the symptoms. The seedlings and adult plants are affected by the pathogen. Eye-shaped spots appear on leaf and leaf sheath having dark brown margin with ashy centre. These spots coalesce to form large patches and the leaves are blighted. Affected leaves wither prematurely and the seedlings may be killed. Lesions appear on the leaf blades, leaf sheaths and culms on adult plants and cause blightening.

Diagnostic Symptoms

• Intially chlorotic (yellowed) lesions and sometimes galls appeared on the upper leaf surface.

• There are corresponding white blister-like dispersal pustules of sporangia on the underside of the leaf.

Ozonium Wilt

Ozonium texanum var. parasiticum Thirum / Guizotia abyssinica

Economic Importance

• The disease was reported around Varanasi.
• Heavy losses are incurred as the diseased plants dry.