Plant Diseases Part I

January 6, 2009

Ah, the signs of spring. The birds are singing, the days are slightly longer, the temperature is warmer, and there are more flowers blooming every day. These are days the gardener has been looking forward to all winter. These are also the days that those dormant diseases have been waiting for. The following is a list of the most common spring diseases to watch out for.

Black Spot: A nasty disease known by all rose lovers. This disease shows up as dark red, or brown to black spots that have a feathered edge on the leaves of its victims. The spots are usually only visible on the upper side of the leaves, the canes, petioles and stipules. This disease does not survive in the soil, but is spread most commonly by splashing water. Consistent attention to regular fungicide treatments, either organic or chemical, should be maintained. Be sure to remove fallen leaves from the area and burn them, or throw them in the trash for disposal to help control the spread of this disease, do not add infected tissue to your compost or yard waste.

Rose Anthracnose: Often mistaken for Black Spot this disease is similar in the fact that it is a fungus and spreads by spores, but the spots are smaller with a definitive edge and can be red, purple, dark brown and black. The leaves affected by this disease show spots on both the top and bottom, often times this will completely eat away the leaf tissue leaving a shot hole appearance, yet the leaves are not as likely to fall off. This disease is very common on roses, but there are several other plants that are susceptible to it. The precautions and treatments are the same as for Black Spot.

Rose Cane Canker: A small yellow, red, or purple blemish that spreads like a large spot girdling the cane causing poor growth or dieback. Remove all deadwood and infected areas. Burn or haul away with the trash, but do not add infected tissue to the compost or yard waste.

Powdery Mildew: This disease effects many plants in our area. You will typically notice a grayish white powdery coating along shoots and leaves. This fungus lives on the outside of the plant surface and spreads root like tissue into the plant to obtain its food. Be cautious of plantings that are naturally susceptible to this disease. A good location with adequate air circulation, good drainage, and more sun are ways to help prevent this disease. The most widely used chemical for control of this disease is a sulfur based spay or powder.

Brown Rot: Most commonly seen on Cherry, Plumb, and Peach trees, this disease will cause a sudden wilting and browning of the flower parts. The flowers will soon be covered by a grayish brown powder that is actually the fruiting fungus. If you have fruiting trees the flowers will not be able to pollinate. If your tree is able to produce fruit, it may show up later on this fruit as well and the leaves and twigs. The twigs will show signs of the Brown Rot by way of an oval canker with a definite outline and the twig will sometimes girdle and die. The fungus will over winter in infected fruits left on the tree or ground, and infected twigs. Use fungicides on infected tissue and clean up the area thoroughly in the fall. Proper pruning of diseased tissues and providing better air circulation will also help to prevent further outbreaks.

Fire Blight: One of the most destructive diseases to infect plants. You can recognize this by the very description of the disease. The flowers, leaves, and branches will appear as though a torch was taken to them, they will be curled in, wilted and black. If you follow the visibly infected area down the branch, you will find an open wound that looks like a pussy canker (YUK!). Use the lower edge of this canker as your guide to measure the minimal 15″-18″ of branch that must be pruned away. The bacterial ooze is spread to blossoms, twigs, and branches by pruning tools that are not sterilized between each cut, insects, and rain. Preventing succulent growth and over production of shoots will help prohibit the development of Fire Blight. There are a few protective sprays that may help prevent blossom infection, but once the disease is there, removal of infected tissue is the only cure. Follow this by sever sanitary measures. All infected parts should be removed from the area and burned. Be sure to sterilize any cutting tool between each cut with a solution of one part bleach and five parts water.

Not all plant problems are from diseases. Be sure to look for the obvious indications of fungus, bacteria, or virus. If you don’t find these clues, look for other problems such as pests, winter damage, root damage, soil compaction, poor or over fertilization, too much or not enough water, improper planting conditions such as planted too deeply, and possibly too much or not enough sunlight. One other over looked problem for many plants used in this area is sodium vapor street or security lighting. If possible don’t plant susceptible plants under these lights, or change the light to a mercury of fluorescent lamp.

When working with diseased plants, always follow sterilization practices to prevent further spreading of the problem. Look for early signs, and treat immediately. Living in Western Washington allows us to grow a wonderful variety of plants, but it’s up to us to keep those plants healthy during our excessively moist springs. Grow on!

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