Mildew on Roses Have Common Link

Q. My roses are covered with mildew.  I have sprayed them and hope they will recover.  I have not had this problem in years.  What causes mildew and how do you keep it from coming back?

Powdery mildew of roses, severe

A. Diseases develop on plants when the environment is just right for that disease, disease organisms come in contact with the plant and the plant must be susceptible to it.

Let’s break each of these factors down. Remember, when dealing with diseases it is most important to prevent diseases from happening rather than cure them with a pesticide after the fact.

Powdery mildew of roses, or powdery mildew of any plant for that matter, develops best in shady spots during cooler times of the year and when it is moist. It does not like to develop in full sun and when it is hot and dry.

We see powdery mildew during cool weather of spring and fall and times just after it rains. It is common when roses are grown in shady spots and watered with sprinklers.

Powdery mildew of weeds

Powdery mildew can be spread by splashing water. This can be from rain or sprinklers. It can also be spread by hands, pruning shears and wind.

Powdery mildew likes some roses better than others. The variety of rose and how healthy it is has a lot to do with how often you will see powdery mildew on it.

So if you want powdery mildew on roses then grow a variety susceptible to powdery mildew, don’t fertilize it very often, prune it incorrectly, place it in the shade and water it with sprinklers.

The flip side is best for disease free roses: select a variety resistant to powdery mildew, plant it in at least 6 to 8 hours of full sun each day, apply compost and wood mulch to the soil, keep it healthy with at least one application of a rose fertilizer every year and irrigate using drip irrigation.

These links may help you with some general information on powdery mildew.

http://xtremehorticulture.blogspot.com/2014/12/euonymus-plants-have-white-spots-and.html

http://xtremehorticulture.blogspot.com/2015/01/pick-right-variety-of-roses-for-desert.html

Snails, Disease and Shade Are Related

Q. I have been noticing tiny little snails climbing up my red brick planter during the fall. I have thousands of those shells all through my planters and yard. My roses have now developed powdery mildew disease which they never have in the past. My lawn is thinning I think because of the snails. I have shade in my yard because of some older mulberries. I sent you some pictures of my yard.

Readers yard with shade

A. After seeing the pictures and reading your description I think that the shade is contributing to a number of things going on. Increased shade causes plants and the soil to stay wet longer.

Snail shells from another readers question

Staying wet longer favors snails and slugs. Increased shade increases the probability of powdery mildew and other diseases. Increased shade causes lawns to thin and eventually fail as well.

          You will see an improvement in everything if you remove some lower limbs of the trees. Limb removal will allow more light on your property and improve the roses, reduce disease problems, thicken the lawn and reduce snail problems.

Cool season lawn grasses like fescue and ryegrass needs direct sunlight at least five hours a day or filtered sunlight so that no more than 50% of the lawn is shaded. Shading lawns and flowering plants more than this is going to hurt them.

Shady lawns are not vigorous enough to withstand any kind of traffic. The lawn will thin and bare soil will appear in the more shaded areas.

Plants that flower, like roses, have fewer blooms and the blooms will be poor quality in the shade. Powdery mildew loves the shade. Powdery mildew also likes splashing water. If there is overhead irrigation that is splashing on the leaves of roses and they are shaded, it will spread powdery mildew from rose to rose.

          Snails are difficult to control. The usual control methods are trapping and baiting. Trap snails by placing wet newspapers or cardboard between the plants. When the sun comes up, snails and slugs like to have parties under wet paper or cardboard.

Powdery mildew on rose

Pick them off of the underside of the cardboard or from the ground and put them in a plastic bag for disposal. If you do this on a regular basis, say weekly, you will start to put a dent in their population.

Snail and slug baits also works well on snails. These are typically spread around the plants periodically and according to the label. These can be purchased in most nurseries and garden centers or online.