Diseases of Vegetable, Ornamental and Spice Crops

DISEASES 2

5. WHITE ROT
Symptoms:

• The fungus invades roots and the basal part of the bulb scales.
• The first symptoms of the disease appear as yellowing and wilting of the leaves followed by a total collapse of the tops.
• If the diseased plants are pulled gently, they will come up readily because of the rotting of centre roots and affected parts are covered with a thick white mycelial mat bearing numerous small sclerotia.

Pathogen:

• The disease is incited by Sclerotium cepivorum Berk.
• The fungus produces sclerotia which are uniformally round measuring 0.35 to 0.50 mm in size.
• Sclerotia germinate only once.

Disease cycle and epidemiology:

• The fungus is mainly disseminated by bulbs which have come from infected soil.
• The fungus persists in the soil and remains viable indefinitely.
• Disease develops rapidly in the soil having 40 per cent water holding capacity.
• Sclerotial germination occurs at temperatures ranging from 9 to 24^o C with optimum being 14 to 18^o C with low moisture.

Management:

• Collect and destroy the infected plant debris.
• Removal of infected plants during season reduces the sclerotial population and also avoids incorporation of the same in the soil.
• Soil solarization during summer months also reduces the incidence of this disease.
• Incorporation of Trichoderma harzianum in soil after solarization also controls S. cepivorum in soil effectively(40 g/m2).
• Several fungicides have been advocated but Iprodione was found most effective in containing this disease.

6. ONION SMUT
Symptoms:

• The first symptoms of the disease appear on the cotyledons as dark, thickened areas on the surface which involve one to several millimeter of the surface.
• The seedlings often die before emergence (Plate -6).
• Large lesions can cause leaves to curve downward (Plate -6).
• Mature lesions exposed, black, powdery spore masses (teliospores).
• Infection progresses inward from leaf to leaf, and infected plants become stunted and may die within 3 to 4 weeks after emergence.
• If the plant survives, the disease becomes systemic and the plant remains in vegetative phase for the whole growing season.
• Bulbs are also covered with blackish lesions.

Pathogen:

• The disease is caused by Urocystis cepulae Frost.
• The sori of fungus looks dark coloured spore masses.
• Chlamydospores or smut teliospores are single celled, spherical or ellipsoid, reddish brown, smooth, thick walled, surrounded by a layer of small, slightly coloured, sterile cells and germinate by means of short promycelium while still held in ball.

Disease cycle and epidemiology:

• The pathogen is soil borne and perpetuates in the form of chlamydospores.
• If the host is available, the spores germinate immediately otherwise they remain dormant indefinitely.
• Optimum temperature for spore germination and growth is 13 to 22o C.

Management:

• Use healthy seed and treat them with thiram or captan (0.3%).
• Before sowing, the nursery bed should be treated with Formaldehyde (5%).
• Avoid the raising of nursery at same location every year.
• No resistance source has been reported in onion but does occur in other Allium spp. which can be used in breeding programme.

7. ONION BLAST
Symptoms:

• The symptoms of the disease appear as white specks with necrotic centres surrounded by a light green halo and lesions may be either isolated and few in number or numerous (Plate -7).
• They may expand slightly with age and take on an elliptical shape and the halo may disappear.
• Many lesions remain restricted in size but under prolonged moist conditions, the fungus develops rapidly and causes blighting of leaves.
• Such leaves are killed prematurely resulting in reduction of bulb yield.

Pathogen:

• The disease is caused by several species of Botrytis like B. allii Munn., B. byssoidea Walker, B. squamosa Walker, B. cinerea Pers. and B. cepae Hanzawa.
• Botrytis is characterized by its hyaline and septate conidiophores which arise as branches of the mycelium with side branches at the tips, each of which has many ampullae that swell gradually at the tips to form conidia on fine denticles.
• Conidia are hyaline or tinted, single celled and globose to ovoid.
• Sclerotia may form on leaf debris or on the necks of onion bulb and germinate by forming stipes on which conidiophore and conidia are produced.

Disease cycle and epidemiology:

• The fungus perpetuates as sclerotia or mycelia in crop debris.
• Sclerotia upon germination produce conidia at 3 to 27^o C (optimum at 9^o C) and serve as source of primary inoculum.

Management:

• Collect and destroy the infected plant debris.
• Follow crop rotation avoiding Allium spp. in rotation.
• With the initiation of the disease, spray the crop with captan (0.25%) or mancozeb (0.25%) and repeat at 10-14 days interval.

8. ONION SMUDGE

J.C.Walker (1929) gave biochemical resistance i.e. red onion cv. posses catechol and protocatechuic acid that provide resistance against onion smudge pathogen. This disease is more prevalent in white coloured onion var. grown in temperate region of the world. It is one of the important diseases in storage.

Symptoms:

• In seed bed it causes damping off of seedling.
• Disease is characterized by the appearance of dark green to black smudge (minute stromata of the fungus) on bulb, neck or green leaves.
• In humid climate pinkish masses of spore can be seen.

Pathogen

• Disease is caused by Colletotrichum circinans (Berk.) Volino.
• Acervuli are formed on stromata just beneath the cuticle by formation of pallisade layer of short conidiophores along with dark satae.
• Conidia are fusiform, falcate and hyaline.

Disease cycle and epidemiology:

• Pathogen survives in stromata and as a saprophytic mycelium in the debris.
• Stromata give rise to acervuli and conidia. Disease development takes place at 10-32ºC but optimum being 26ºC.
• Moist condition favours the production of conidia.

Management:

• Grow coloured variety to minimize disease incidence.
• Dry the bulbs before storage for curing.
• Foliar application of mancozeb (0.25%) in combination with carbendazim (0.05%) will reduce disease development.

Other diseases of importance are:

i. Rust: Puccinia asparagi DC.
ii. Pink rot : Pyrenochaeta terrestris (Hansen) Gorenz et al.
iii. Black mould: Aspergillus niger V.Tiegh.
iv. Bacterial blight: Pseudomonas gladioli pv. allicola
v. Onion yellow dwarf : Onion Yellow Dwarf Virus
vi. Garlic mosaic : Garlic Mosaic Virus
vii. Aster Yellows : Phytoplasma