Black knot of plums and cherries is a widespread and serious disease throughout North America. The black knot fungus mainly affects twigs, branches, and fruit spurs. Although sometimes trunks may also become diseased. Usually, infections begin on the youngest growth. On infected plant parts, abnormal growth of bark and wood tissues produce small, light-brown swellings that eventually rupture as they enlarge. Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus. The entire tree may gradually weaken and die if the severity of the disease increases and no effective control measures are taken. Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored.
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Silver Leaf is a fungal disease and its name refers to the silver luster of leaves that occurs on some hosts. However, not all trees show silver foliage when infected, but rather the first sign of the disease is twig and branch die-back. Leaves that are affected may later start to show brown areas in the middle and at the edges. Toxins produced by the fungus affect leaves, and on some hosts it may kill branches or entire trees. Diagnosis can be confirmed by cutting through a branch that is at least 2.5cm (1in) in diameter, wetting the cut surface and checking it for a brown stain in the wood. To control the disease, prune at the first sign of silvering. Make the cut below the diseased area into healthy wood. Cover pruning wounds that are larger than 1.5cm with pruning sealer. Remove and bury or burn all infected branches, leaves and twigs. This will help to reduce the amount of disease the following year. No chemical control is available.
This fungal disease attacks leaves and twigs. Symptoms include white patches on new leaves, severely affected leaves may start to curl and drop prematurely. Rake remove and destroy any fallen leaves. High humidity and intermittent rains make ideal conditions for the fungus. This disease can be managed by spraying with a fungicide. Proper tree pruning can reduce the chance of infection of Powdery Mildew by creating an unfavourable environment for the fungus.
Cherry Leaf Spot
The fungus that causes this disease usually attacks the leaves but may affect twigs and stems. Leaf Spot occurs in many parts of North America and the world where humid conditions occur. Symptoms include small dark coloured spots on the leaves and the leaves may begin to yellow while areas around the spots may remain green. Some leaves may drop prematurely. During wet weather white patches may appear on the undersides of leaves at the centre of the dark spots. These white patches contain fungal spores. The disease overwinters when infected leaves fall to the ground. Wind and rain splash the spores up onto the tree's new leaves and buds in the spring. To manage Leaf Spot, it is recommended to spray with a fungicide. Rake and destroy any fallen leaves to help reduce the amount of disease the following year.
The fungus that causes this disease, can infect fruit, blossoms and small branches. Symptoms include blight, cankers and fruit rot. Cherry fruits that are affected rot very quickly, they shrivel and become covered in a brownish-gray substance. Powdery tufts of brown gray spores are visible on the outside of infected flower shucks, and on infected fruit or twig surfaces especially under wet weather conditions. The dried infected fruits are called mummies as a result of their appearance. Wind and rain spread the spores from infected mummies and twigs to uninfected parts of the tree. Mummified fruit and cankers should be pruned during the dormant season and either burned or buried deeply in the soil. Remove and destroy all dropped and rotted fruit from the ground beneath the tree. Blossom blight is much more serious on sweet cherry than on sour cherry. Sweet cherries (not sour) are also very susceptible to infection the first few weeks after fruit set, and a petal fall spray is recommended if the weather is warm and wet. Superior brown rot fungicides should be used on sweet cherries three weeks before the fruits are ripe.
Canker is caused by two related bacteria. The symptoms on the leaves are similar to those of leaf spot but the two diseases are distinguished by the cankers on trunks, limbs and branches that exude gum during late spring and summer in the case of Bacterial Canker. Spots on the leaves are dark brown, circular to angular, and sometimes surrounded with yellow. Leaves that are on the terminal side of cankers may wilt and die. Lesions on green cherry fruit are brown with an area of wet tissue, as well black depressions may be left on the skin of the fruit. Outbreaks are common following cool wet weather after bloom. Prune out diseased branches in late winter or early spring. The bacteria can be transmitted by pruning tools so these should be disinfected between trees if bacterial canker is present. Spray with a fungicide that contains basic copper sulfate in the fall. However, copper sprays can cause injury to stone fruit.
Black Cherry Aphids
This problem is reported wherever cherry trees are grown. Black Cherry Aphids are the only type of aphids that affect cherry trees. The aphids are small soft-bodied insects about 3mm in length and may or may not have wings. Affected leaves become stunted, twisted and curled. The Honeydew secreted by the aphids drips on the leaves, fruits and surrounding ground. Ants may be present. A black fungus begins to grow on the honeydew where it has fallen. Treatment includes spraying with an insecticide to kill the aphids.
Small holes may appear in the trunk from which a gummy sap oozes. During the growing season some of the leaves and branches may wilt and die, turning brown. Since borers usually attack weakened or stressed trees, it is important to maintain their health with enough water and proper fertilizer. There are two types of borers that affect cherry trees; the Shothole Borer and the Peachtree Borer. The tunnels created by the borers interfere with the flow of water and nutrients within the tree. Keep the tree healthy by watering regularly and applying a fertilizer. The trunk and large branches can be sprayed with an insecticide designed to kill borers, however they can be very difficult to control. Contact a professional arborist.
San Jose Scales
Symptoms may include leaves that are pale green or yellow and drop or they may turn brown and stay on the tree into the winter months. The fruit may be covered with specks. Small, hard, gray raised bumps encrust the bark. If left uncontrolled large branches may be killed after two or three seasons. To treat, spray the trunk and branches with horticultural oil during the dormant season in early spring.
Additional insects to be aware of include Cherry Fruit Flies, Pearslugs, Tent Caterpillars and the Oriental Fruit Moth. Treat these problems with an insecticide and for caterpillars also use a bug band.
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