- It is widespread in those regions of the world, which have cool and wet climate.
- 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.
Disease cycle and epidemiology
- 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.
- 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 20-24oC, respectively.
- 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.
Last modified: Monday, 12 March 2012, 6:06 AM