All you need to know about our enemy, the powdery mildew!

Powdery mildew is a common disease on many types of plants. Although different species of fungi cause the disease on different plants (Erysiphe infects vegetable crops and flowers; Podosphaera species infects apples and stone fruits; Sphaerotheca species infects berries, roses, some vegetable crops, and stone fruits; and Uncinula necator infects grapes), the infections are all characterized by a powdery white to gray fungal growth on leaves, stems and heads.

Symptoms

Powdery mildew usually shows up on leaf and stem surfaces, looking as if baby powder was sprinkled over them. Woody species such as grapes, fruit trees, roses, crape myrtle, and sycamore can be seriously affected; new growth is often distorted. The young fruit of apples and grapes can also develop rough skin due to powdery mildew.

Life Cycle

All species of powdery mildew fungi require living plant tissue to grow. On perennials, they survive on buds and stem tissue. Certain weeds will also act as hosts through the winter. The optimum temperature for infection is between 68 to 77 degrees F and relative humidity between 40 to 100% is sufficient for the spores to germinate. Low, diffuse light also seems to favor powdery mildew development.

The mildew can spread rapidly, in as little as 72 hours. However, it commonly takes 7-10 days from the time of infection to the development of symptoms and secondary spore production.

Management
In most cases, good cultural practices will adequately control powdery mildew:

1.     Select powdery mildew resistant varieties. This is particularly true of roses. For lawns, shade tolerant grasses such as creeping red fescue can be planted.

2.     Plant in full sunlight in a well-drained area.

3.     Do not crowd plants. Air flow and ventilation will discourage mildew growth.

4.     Powdery mildew thrives where high rates of nitrogen have been used. High nitrogen promotes tender leaf formation, causing dense stands that are more susceptible to infections. Adequately fertilize but avoid stimulating succulent growth. Organic fertilizers or slow-release formulations of lawn fertilizers are good choices.

5.     Prune infected plants to get rid of infected parts and increase airflow. If the infestations are severe, remove and destroy the plants that are infected.

6.     Disinfect your pruning tool in a bleach solution of one part household bleach to four parts water after each cut.

7.     Watering plants in the morning gives the plants the rest of the day to dry off, discouraging establishment of diseases, including powdery mildew.

Organic Sprays

In 1999, a Brazilian scientist named Wagner Bettiol discovered a new alternative to controlling powdery mildew - weekly sprays of milk!. Not only was milk found to be effective at controlling the disease, it also acted as a foliar fertilizer, boosting the plant's immune system.

How does milk control powdery mildew?

Scientist aren't 100% sure how milk works to control this disease. It seems that milk is a natural germicide. In addition, it contains several naturally occurring salts and amino acids that are taken up by the plant. From previous experiments using sodium bicarbonate, potassium phosphate, and other salts, researchers have found that the disease is sensitive to these salts. It is possible then, that milk boosts the plant's immune system to prevent the disease.

What kind of milk should be used?

In Bettiol's original experiment, fresh milk was used, straight from the cow. However, this is obviously not feasible to most home gardeners. The research work in New Zealand actually found that using skim milk was just as effective. Not only was it cheaper, but the fact that the milk had no fat content meant that there was less chance of any odours.

Sulfur

Sulfur is also  highly effective against powdery mildew if used in a protectant program with a minimum of 7 to 14 days between applications. Apply a sulfur-based fungicide at first evidence of mildew and repeat applications as necessary. Proper timing is critical to successful control so make sure to begin at the first sign of the disease.

Baking Soda

However, sulfur can be damaging to some squash and melon varieties. Another option is to spray once a week with a solution of baking soda. Baking soda increases the surface pH of the leaf making it unsuitable for the growth of powdery mildew spores.

Be sure to spray the undersides of leaves as well as the upper surfaces when using any of these sprays.

Here's a recipe to make your own spray:

1 teaspoon baking soda
1 quart water
A few drops of liquid soap

Before treating your plants, test the spray on a few leaves to make sure they are not too sensitive.