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Hebe Downy Mildew


Hebe Downy Mildew

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Caused By:-     The Fungus Peronospora grisea


Hebe Downy Mildew is caused by the fungus Pseudoperonospora griesea and is not a true fungi but is more closely related to algae. Downy mildew will not usually kill the Hebe as it needs a living host to survive and reproduce. The growth is felt like and is purplish-brown in colour. There is usually some yellowing and blotchiness on the upper leaf surface along with some leaf curling at the edges of the leaf.
The majority of the growth of downy mildew occurs within the leaf tissues and the felt-like production of spores on the bottom of the leaf has very angular patterns that follow the lines of veins. The infected leaves may eventually die and fall.
When conditions are right, with cool, moist conditions, transmission is by asexual spores in spring and summer. These develop into the hypae which come together to form the mycelium that produces sexual spores. 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.
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.
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.
See also Downy Mildews Brassica, Downy, 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.


The best method of control is prevention therefore good hygiene and cultural practices are essential to avoid this disease.
Raking up fallen leaves, placing plants in full sun, Winter tar, oil or fungicide wash and pruning out severely infected branches and burning them will all help in preventing attack from these fungi.
Control of leaf wetness and humidity, by allowing good airflow through and around the Hebe, is important if attacks from downy mildew is to be avoided. Fungicides are often ineffective and special systemic fungicides will have to be used.
A weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water) is said to significantly reduced the severity of some mildew infection.
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 powdery 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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