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

DOWNY MILDEW

• It is widespread in those regions of the world, which have cool and wet climate.

Symptoms:

• The first symptoms of the disease are evident as small brown spots on the lower surfaces of the leaves.
• The spots are covered gradually with the downy growth consisting of conidiophores and conidia of the fungus (Plate-1a).
• Upper surface of the leaves corresponding to spots turns light yellowish in colour (Plate-1b).
• Young leaves having a number of such spots turn yellow and drop off early whereas older leaves usually persist.
• The deformities are also seen on inflorescence where flowering stalk thickens and becomes elongated and curved.
• Black sunken spots are also produced on cauliflower and broccoli curds as well as on cabbage heads.

Pathogen:

• The fungus responsible for this disease is Peronospora parasitica (Pers.) Fr. which is an obligate parasite.
• The mycelium of the fungus is strictly intercellular with large, finger-shaped intracellular haustoria which become clavate and branched and nearly fill the cell cavity.
• The sporangiophores are produced during darkness, which are divided into primary and secondary branches (dichotomously branched), which ultimately bifurcate to form sterigmata which bear the single sporangium at the tip.
• The sporangia are broadly oval, ellipsoidal and hyaline and they fall off mainly by hygroscopic twisting of the sporangiophores in response to change in atmospheric humidity.
• The oospores are globose measuring 26 to 43 µm in diameter.
• They are enclosed in crest like folds and appear pale yellow in colour and germinate by germ tube.

Disease cycle and epidemiology

• The downy mildew pathogen perennates as oospores in senescence host tissues and Albugo candida induced malformed inflorescence (Fig.1.).
• It also survives as conidia on leaves and inflorescence and as latent systemic mycelium in seeds or infected plant debris.
• Infections are favoured by low temperatures and high atmospheric humidity following rain or dew.
• The penetration is usually direct but occasionally also occurs through stomata.
• Primary infection occurs due to soil borne oospores while conidia released from conidiophores found on the cotyledons or hypocotyls favour secondary spread.
• Water droplets also help the pathogen dispersal over short distances.
• The optimum temperature for conidial germination is 8 – 12 ^o C while penetration of the host by infection hyphae and formation of the haustoria occurred most rapidly at 16o and 20-24^oC, respectively.

Management:

• Collect and destroy the infected plant debris and perennial weed hosts.
• Crop rotation with non cruciferous plants.
• Use disease free seeds. Heat treatment of seeds at 43 – 50o C for 20 minutes and treating them with Apron metalaxyl + mancozeb (0.3%) is also effective.
• Spray the crop with metalaxyl + mancozeb (0.25%) or mancozeb (0.25%) or copper oxychloride (0.3%) and repeat at 10 to 14 days interval.