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Downy Mildew



P_downymildew

Downy Mildew

Usually appear in

Spring to Summer


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Caused By:-     Mainly Peronospora fungus species

Description

Downy mildews are a common plant disease. It can affect all kinds of plants. Downy mildew is often confused with powdery mildew, but they are totally different diseases. Downy mildew is not a true fungi but is more closely related to algae and the symptoms are different.
This is a fungal disease that grows in cool, moist conditions and appears as a dirty-white fluffy growth under the leaves. It is caused by caused by several fungi, including species of Basidiophora, Bremia, Peronospora, Phytophthora, Plasmopara, Pseudoperonospora, and Sclerospora. It does most damage to young plants in spring. Plants attacked by this fungus include Brassicas, Geraniums, impatients, lettuce and onions. The symptoms can be mistaken for the Powdery Mildew fungus. When conditions are right transmission is by asexual spores in spring and summer. These develop into the hypae which come together to form the mycelium. Sexual spores can survive on fallen leaves until conditions are right. The fungus creates infected areas on the leaves which are angular. The spores resides in plant debris which carry it through winter to infect leaves with fungal mycelium early in the season. These produce more spores which are wind or water splash borne to infect other plants or leaves. The effect of this fungus is to reduce the vigour of the plant, which can eventually die.
It is associated with the a high relative humidity weather conditions at that time. Therefore, Crowded plantings where air circulation is poor damp and in shaded areas makes the problem worse. Young growth usually is more susceptible than older plant tissues.
When the infected leaf tissues eventually die, it forms resting spores in the dead material which will then contaminate the soil.
If it is sprayed in the early stages only some of the tissues will die and the leaves will be scarred. The youngest leaves and shoots are especially susceptible to the fungus. The white growth on the leaves are the spores which are carried by the wind or plashes from watering.
Downy mildews are usually host specific, which means they do not spread to other types of plants. There are many types of Downy Mildew fungi and they all produce similar symptoms usually on the undersides of leaves. Infected leaves may become distorted, turn yellow with patches of green, and fall prematurely. The infected areas dies or may become soggy.
Picture by Charles Averre, North Carolina State University, Bugwood.org See also Downy Mildews Brassica, Hebe, Lettuce, Onion, Pansy, Pea.
See also other Mildews American Gooseberry, European Gooseberry, Gooseberry.
See also these other Powdery Mildews Apple, Begonia, Hydrangea, Oak, Pansy, Pea, Phlox, Powdery, Quince, Rose.


Control

Cultural control is by growing plants in good conditions, which include plant separation to allow a good air flow, avoiding overwatering, wetting the leaves when watering and poorly drained soil, the removal of plant debris which can carry spores and weeds which can act as alternate hosts, eg. Shepherds Purse in the case of Brassicas, rotating annual flowers and vegetables, removing and destroying diseased parts of the plants.
Other avoidance measures are growing resistant varieties, sowing disease-free seed and maintaining balanced soil fertility. All these good husbandry methods discourage the growth of the fungus.
The application of copper carbamate fungicides maneb, or zineb is effective against many downy mildews, but the amount of residue on vegetables must be taken into account.
Weekly preventative sprays of baking soda( 1 tablespoon of baking soda a few drops of washing up liquid to 1 Gallon of water) makes an inexpensive control for many mildew on plants. Once the infection has taken hold baking soda offers only minimal benefits.
Some plants can be burnt by the baking soda therefore test before use on specific plants.
Spraying with a mixture of baking soda(sodium bicarbonate) or potassium bicarbonate combined with a lightweight horticultural oil is said to cure and prevent the problem.
Mulching the plants should help in preventing many mildews.



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