Native to parts of the Southeastern United States, the shrub species Viburnum acerfolium is known for its maplelike leaves, earning it the common name of maple-leaf viburnum. This species grows up to 6 feet tall and 4 feet wide, bearing flat-topped clusters of white flowers in late spring and black berries during late summer. Viburnum acerfolium plants are most often affected by bacterial and fungal leaf spots, but they're also susceptible to several other diseases. Maple-leaf viburnums are most at risk for diseases when they're under stress or planted in nutrient-poor soil.
Fungal Leaf Spots
Caused by several different species of fungi, fungal leaf spots are irregular-shaped, reddish to greyish-brown that are usually dry and sunken. Fungal leaf spots most often arise on maple-leaf viburnum shrubs during summer when the weather is moist and warm. You can treat these diseases by improving the air circulation around the viburnum and keeping the foliage as dry as possible. Selective pruning of overhanging branches and crowded growth can help keep the fungi from spreading. Also, rake up fallen diseased leaves and debris, and then destroy them to prevent the spread of the fungal disease. If your Viburnum acerfolium is severely infected with fungal leaf spots, you can apply a fungicide like mancozeb, thiophanate-methyl or chlorothalonil every 10 days to two weeks until the symptoms disappear. Algal leaf spot, which is caused by the fungus Cephaleuros virescens, occurs during moist, cool weather and causes pale-green leaf spots that turn light-brown to reddish-brown. The spots have feathered margins and are raised. This type of leaf-spot disease is best controlled using a copper fungicide, applied once every 10 days to two weeks until the symptoms clear up.
Caused by the fungus Erysiphe sparsa, powdery mildew is another fungal disease that affects the maple-leaf viburnum's foliage. This disease is usually easy to recognise by the white, powdery fungal-spore growth that covers the leaves, stems and flowers. You might also notice that new growth on your viburnum is stunted or misshapen. Treat powdery mildew by spraying your Viburnum acerfolium with a fungicide containing triforine, thiophanate-methyle, myclobutanil, propiconazole or triadimefon. Applications of horticultural oils are alternatives to fungicide chemicals, but you can't use them if air temperatures are warmer than 29.4 degrees Celsius.
Bacterial Blight & Canker
Bacterial blight is caused by the strain Pseudomonas syringae and occurs during humid, rainy weather in spring. Bacterial blight and canker can cause black angular lesions or brown spots on the Viburnum acerfolium's leaves, along with stem and flower dieback. You might also see lesions on the twigs and other woody shrub parts, which might ooze a gummy or saplike substance. Although no effective chemical treatment is available for bacterial blight, you can sometimes control the disease by pruning away all symptomatic shoots, branches and other growth from the viburnum. Avoid overfertilizing and overhead watering, and perform your pruning during the drier months, to help treat and prevent the spread of bacterial blight and canker.
Phytophthora Root Rot
Collar, root, crown and foot rots caused by the Phytophthora species of fungi can cause your maple-leaf viburnum's leaves to become dwarfed and stunted, and then wilt and drop early. These diseases cause symptoms on the bark and wood as well, including reddish-black sap oozing from the bark, cankers or vertical stains on the trunk, and discolouration of the wood beneath the bark. Phytophthora rots can eventually cause twig and branch dieback, followed by death of the entire shrub. Good cultural practices, such as improving soil drainage and not overwatering your viburnum, are the best remedies against Phytophthora rots. You might also try spraying the maple-leaf viburnum with a fungicide containing fosetyl-al, often marketed under the brand name Aliette.
Armillaria Root Rot
Another root-rot disease that affects Viburnum acerfolium shrubs is caused by the fungus Armillaria mellea. Also called shoestring root rot, this disease causes stunting, yellowing and dropping of the leaves, as well as white fungal growth beneath the bark on the lower trunk or "crown." Armillaria root rot also causes black strings of fungal growth beneath the bark, on the upper roots and in the soil around the infected viburnum. No effective treatment for Armillaria root rot exists, but you can prevent and lessen the fungal infection by fertilising the viburnum properly, watering the shrub during droughts and providing good soil drainage. If you maple-leaf viburnum's roots are severely infected with Armillaria fungus, you may need to remove the shrub and destroy all the plant parts, including the surrounding soil.
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