Diseases of Vegetable, Ornamental and Spice Crops (2+1)

WHITE RUST OR BLISTER

It is the most common disease of radish and other crucifers and cause considerable losses when it occurs along with downy mildew.

Symptoms:

• Two types of infection i. e. local or leaf phase and the systemic or stag head phase occur in different crucifers.
• Local infection results in isolated pustules on leaves and stems which show characteristic raised, glossy white blisters 1 to 2 mm in diameter.
• Systemic infection stimulates hypertrophy and hyperplasia resulting in enlarged and variously distorted organs (Plate-4).
• The inflorescence may become enormously thickened, fleshy and greenish leading to sterility.
• The thickened inflorescence is generally known as stag head.
• Blisters after rupturing the host epidermis release white powdery mass comprising of sporangia.

Pathogen:

• The disease is cuased by Albugo candida (Pers. Ex Chev.) Kuntze.
• It is an obligate parasite reproducing both sexually and asexually.
• Mycelium is intercellular and non-septate.
• Numerous short conidiophores arise from the mycelium, which are arranged in rows beneath the host epidermal layer press on the epidermis to cause the pustule/ blister.
• Inside the pustules, sporangia are produced in chains arising from the sporangiophores at the base of the cavity.
• The sporangia are hyaline, spherical, thick walled, hyaline and interconnected by a pad of gelatinous disc like tissue.
• In the presence of moisture, these pads swell and disintegrate, freeing the sporangia in the chains and they germinate either by germtube or by producing zoospores.
• The oospore is spherical with a thick irregular wall, deep yellow to dark brown in colour.

Disease cycle and epidemiology:

• The fungus survives either as oospores in the infected plant debris or mixed with seeds or as perennial mycelium in some weed hosts (Fig.3).
• These oospores may germinate in the ensuing crop season to produce zoospores, which serve as primary inoculum.
• Secondary spread is through sporangia produced on the spots caused by primary inoculum.
• Moisture is essential for germination of sporangia through zoospores.
• ? The optimum temperature for germination of sporangia by zoospores is 20^o C.

Management:

• Practices like collection and burning of diseased plant material in order to prevent oospore formation, reduction of humid conditions around the plants by maintaining proper spacing, crucifer weed free cultivation, application of fertilizers like phosphorus, potassium and avoidance of excess application of nitrogen keep the disease under check.
• With the initiation of the disease, spray the crop with metalaxyl + mancozeb (0.25%) followed by sprays of Bordeaux mixture (4:4:50) or mancozeb (0.25%) or copper oxychloride (0.30%) and repeat at 10 to 14 days interval.