Be on the lookout for plant disease after that rain

Susan Richards unknown / jpg, SH

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With the return of rain after such hot, humid weather, it is time to look for disease in the garden.
It is much easier to prevent plant disease than cure it. If you keep plants healthy with regular fertilizing; plant and trim so that there is good air flow and clean up any damaged leaves as soon as you notice them, you will be well on your way to a healthy garden.
Some common diseases that you need to watch out for:
• Powdery Mildew is one of the most common leaf diseases. It shows up as a greyish-white film on the leaf surface that can be easily wiped off. It thrives in hot humid or cool damp weather.
Prevention is the best cure for this problem.
Water early in the day so foliage is not wet overnight. Treat disease prone plants with a spray or dust of Garden Sulphur every week to 10 days. Tall garden Phlox, begonias, cucumbers and squash are a few of the plants to watch.
• Black Knot is another common disease that is extremely easy to spot. Members of the Plum family, Chokecherry and Pin Cherry are very susceptible.
You will see a black bulbous growth of tissue emerge from a crack in the bark of branches or tree trunks. Quick pruning is the only treatment. Cut the limb off 12 inches below the diseased area, bag the branch and get it off your property.
Dip your pruning tools in rubbing alcohol after each cut so that you don’t spread the disease around.
After pruning, spray the plant with a Copper Sulphate fungicide. If you have Black Knot in the trunk of a tree, unfortunately it must be cut down.
• Black Spot is a common leaf disease. Many roses are prone to it.
A black sooty spot appears on the leaves; the leaf turns yellow and falls off.
Using sulphur as a preventative spray is always the best way to combat this problem. In cool, damp weather, keep an eye out for leaf spots on Flowering Crab Apple trees too. These trees have been hit hard the last few years.
Once again watch your watering practices and clean up, then bag any diseased leaves. Do not put them in the compost pile.
• Clematis Wilt is a disease that can hit suddenly and kill an entire plant. The fungus is a common one that lives in the soil.
When the stem of the plant gets nicked or cracks, the fungus moves in and rapidly multiplies in the tiny tubes that carry water from the roots to the leaves. The tubes get clogged, the water can’t get through and the leaves suddenly wilt and die.
Be careful when cultivating around your clematis. Try not to damage any stems. Make sure the vines have good support so that they don’t crack on a windy day.
If disease strikes, cut down and bag all stems and foliage. Look closely at the crown of the plant. If it is still healthy, spray with Copper Sulphate and wait to see if it re-grows.
• Fire Blight is another disease that strikes quickly. It can be spread from tree to tree by pollinating insects. Mountain Ash and pear trees are prone to this disease.
Stems will suddenly turn black and leaves look scorched as they quickly die. Prune as mention above for Black Knot and remove the tree if the damage gets into the trunk.
• Leaf Blight is a wide group of diseases that show up on many plants. It shows up as irregular dead spots on the leaves. Copper Sulphate is the best product to use for this problem. It is most common on plants that are stressed.
Try to keep plants well watered during periods of drought. Use the correct fertilizer for the plants you grow and clean up damaged foliage quickly to prevent the spread of the disease.

Columnist Susan Richards is Garden Centre manager of New North Greenhouses at 719 Airport Rd., in Sault Ste. Marie, Ont.

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