Originally created 08/12/05

Rainfall can lead to plant diseases



For the most part, all the rain we have received this summer has been a blessing. We haven't had to water as much, and things look so much greener than normal.

With all this green, we also see our share of problems, including leaf yellowing and reddening, leaf spots that lead to premature leaf drop, and fruit drop. Some of the more common trees affected are maples, dogwoods, pears, river birches and several shrubs.

If you remember the summer of 2003, particularly July, we actually had more rain than we did this year. I visited my mother in Blue Ridge, Ga., in July and she had maple trees on her property that already were one- half to two-thirds defoliated because of leaf spot disease caused by the excess rain.

When soils are saturated, especially in hard clay soils, oxygen can't reach the root system. Some trees are more tolerant of wet conditions than others, but the longer the lack of aeration, the greater the chance of root death. Lack of oxygen will result in root die back, with above- ground symptoms appearing as leaf yellowing, droopy foliage, leaf drop, and eventually branch dieback.

Waterlogged root systems also are more susceptible to attack by root-rot organisms. In addition to the obvious damage to plants, there can be long- term effects to the soil. Soil micro organisms that require oxygen might be killed and those that survive without oxygen might take over, which in turn affects availability of nutrients for plant use.

There isn't much you can do other than wait for drier weather to prevail and allow water to drain. As more favorable conditions return, watch for signs of dieback, but don't be too hasty to cut limbs. Branches that have lost leaves aren't necessarily dead, even though leaves might drop. There might be buds that will be able to re leaf next summer. Live stems and buds will have some green tissue visible.

A light fertilization might be helpful to replace nutrients that were lost and to urge re growth.

Because we have had so many thunder storms this summer, I have had my share of calls about people's trees that have been struck by lightning. Most people assume that they immediately need to cut them down, but this is not the case.

When lightning strikes a tree, it either moves in a narrow line down the branches, stems and roots, or along a wide pathway encompassing the entire tree cylinder. Lightning directly destroys tree tissues by electrical disruption and heat. Steam explosions down the stem, in a wide or narrow band, show where the electrical current has moved through the tree. There also can be massive root damage that is not seen.

Damage caused by extensive lightning leads to excessive water loss, which also is life threatening. Pests can quickly attack a lightning- weakened and damaged tree. For example, pine beetles can quickly destroy a lightning- struck pine.

Only time will tell whether a tree will live when it is struck by lightning. If it dies, it usually will occur within the next six to 12 months. The National Arbor Foundation says that lightning kills about one out of four trees it strikes. This is direct damage.

Indirectly, I would say 75 to 90 percent end up dying because of insects attacking. I used to estimate a higher percentage, but over the years I have seen plenty of trees struck and they are doing fine. There is a pine tree in my neighborhood park that has been struck twice and it is still living.

Sid Mullis is the director of the University of Georgia extension service office in Richmond County. Call 821-2349, or send e-mail to smullis@uga.edu.



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