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Tree Talk - Metropolitan Forestry Services

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May 5, 2016

Watch our for Wet Weather Diseases

It seems like here in Missouri we are constantly faced with weather extremes, and the past few years we have seen extremely wet spring weather. You would think that abundant rain in the spring is a necessity, but these spring showers can also bring big problems for your trees and plants.  In 2015, the St. Louis area spring started with much cooler temperatures and record amounts of rainfall followed by cooler than normal summer with extended drought conditions. These conditions were excellent for the development of fungal and bacterial diseases. Here are some of the most common to watch out for:


Phytophthora Root Rot is a soil born pathogen that infects the roots of weak, stressed or damaged trees. The fungus is always present in all soils but proliferates in heavy or waterlogged soils. Phytophthora affects grass, greenhouse plants, trees and shrubs. In trees, the infection can go unnoticed as it begins in the roots and invades weakened root systems. By the time it appears at the base of a mature tree, much of the root system may already be infected. The vascular system is killed causing a dark oozing with a beer-like fermented smell. Species that are most prone to infection are: shingle oak, red oak, pin oak, beech, red maple, sugar maple and Norway maple, azaleas, rhododendrons, dogwoods, mountain laurels and viburnums. Symptoms to watch for: A thin, declining crown, stunted leaves, and twig dieback can all be signs of a serious problem. Dark oozing spots on lower truck with fermented smell, or increased bee or yellow jacket activity. We have treatment options that work extremely well to combat Phytophthora. Do your best to keep your trees as healthy as possible. Watch watering procedures – DON’T OVER WATER!  Plant trees and shrubs on a raised bed or berm wherever possible, and fertilize to increase vigor.


Anthracnose is a weather-related fungus affecting a wide variety of trees. It proliferates when we have warm days, cool nights and a lot of moisture. Generally, we see this weather pattern in early spring, but Anthracnose can occur any time weather conditions are favorable. The most noticeable signs of Anthracnose are browning, curling and early abscising of leaves from the tree, but it can also cause leaf spots, twig cankers and dieback. If you have a severe case, it can cause major defoliation of your tree. It is rarely fatal, but the continual defoliation can cause stress and open it to up to attacks from insects and other pathogens. We see the majority of infections on ash, sycamore, white oak and maple trees, but it can occur on most any deciduous trees given the right conditions. Treatment is available, but has to be applied preventatively to the leaves before infection occurs. Pruning can help improve air circulation and light. Pick up any leaves that have fallen on the ground, as the infection overwinters in the bark and fallen leaves and can re-infect.


Powdery mildew is a very common fungal disease that affects lilacs, dogwoods, crape myrtles and crabapples, but it can affect most any plants. You will see a white or gray powdery growth form on the leaves, and the plant will begin to look sickly or less vigorous and may begin to drop leaves. Powdery mildew thrives in hot weather and high relative humidity. High humidity encourages spore formation, and low humidity helps disperse the spores. To combat the fungus, space plants appropriately so that air can circulate, remove dead plant material, and do not water with overhead sprinklers. The infection is generally heaviest on foliage that is shaded and has poor air circulation, so placing plants in an area to get at least six hours of sunlight will help tremendously. A fungicide can help to prevent and control the problem.


Two years ago we saw a massive increase in Fire Blight cases in our area. This is a bacterial disease that affects pears, cherries, apples, quince, and hawthorns. This bacterium is spread by birds, insects and rain. Fire Blight is identified by a brown or black burnt look to the leaves, and curling in the shape of a shepherds crook. Since this is a bacterial disease, it is markedly different from the fungal infections mentioned in this article. The treatment protocol consists of three sprays of an agricultural bactericide and copper. The first spray must be applied while the buds and flowers are emerging. The following two sprays are 10-days apart while the leaves emerge depending on weather conditions. This is also a preventative treatment that must be done every year – if you wait until you see signs of the disease; it is already too late to treat. You must be extremely careful when pruning trees with re blight. Timing is critical and pruning instruments should be disinfected between every cut or the bacteria can be spread to healthy wood.


Apple scab is a common fungus affecting apple trees and flowering crabapples. The disease is most prevalent and aggressive in areas where spring weather is mild and rainy. Infection occurs when leaves first emerge and before they reach full size and harden off
which is generally April-May. It begins as small brown spots on the undersides of leaves, and then spreads to the top of the leaf and eventually to the fruit itself. Infected leaves turn yellow and drop rapidly. It appears as light brown spots on the fruit, about 1⁄4 to 1⁄2 inch in diameter. Premature defoliation may occur from late spring through late summer. The Apple Scab fungus overwinters in fallen leaves on the ground and then re- infects that tree in the spring. The spores are spread by wind and rain. It is important that the trees are treated preventatively in the spring time as this is when the infection occurs and not in midsummer when the symptoms begin to appear. Crabapples will require 3-4 applications at 7-14 day intervals in the spring. Proper sanitation, pruning, and watering may help during dry seasons.

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