Figure 1. Anthracnose on inkberry. A wide variety of fungi may cause leaf spot diseases on cultivated holly. In landscapes, the injury to holly may be unsightly but the plants are rarely harmed by these diseases. Often, damage is so light that it's hardly noticeable.
In container and field nurseries, considerable spotting of the leaves and subsequent defoliation often associated with leaf spots and blight diseases may greatly reduce the quality and market value of holly. Typically, outbreaks of leaf spot diseases on most woody ornamentals like holly are associated with extended periods of wet, humid, cloudy weather in the spring and summer.
Cylindrocladium leaf spot, commonly on 'Burford', Japanese, American, and yaupon holly, is seen primarily on container stock in the Southeast. This disease may also be introduced into home landscapes on diseased container stock.
Symptoms first appear as tiny yellow (chlorotic) spots which later enlarge to circular spots with a tan to brown center and a purple-black border. Even a single spot may cause a diseased leaf to be prematurely shed by the plant. On badly defoliated holly, a shoot dieback may also occur.
In nurseries, apply a fungicide (See Table 1) every 10 to 14 days starting at bud break until all new growth matures. In landscapes, apply fungicides only to plants previously damaged by disease. Begin fungicide sprays at leaf-out and repeat every 10 to 14 days until shoot growth has stopped.
Table 1. Chemical Control Of Cylindrocladium Leaf Spot.
The common holly hosts of anthracnose include English, Chinese, American, inkberry, and winterberry holly. Anthracnose may occur on holly in production nurseries and the landscape. Like Cylindrocladium leaf spot, this disease is seen most often on holly across the Southeast.
Circular to irregular blotches usually tan to brown appear on the leaves (Figure 1).
Figure 1. Anthracnose on inkberry.
Numerous pink-orange, pin-head sized spore masses of the causal fungus, that appear during humid weather inside the blotches, can easily be seen with the naked eye or a hand lens. A shoot dieback may also be seen on anthracnose-damaged 'Burford' holly.
Apply a recommended fungicide starting in late spring. Repeat sprays every 7 to 14 days as needed. Recommended fungicides are listed in Table 2. See comments on management of foliar diseases below for other control suggestions.
Table 2. Chemical Control Of Anthracnose.
Outbreaks of web blight or Rhizoctonia aerial blight are seen most often on rooted liners and container-grown dwarf cultivars of Japanese and yaupon holly, though liners of other hollies may also be damaged. In general, holly cultivars with dense canopies that are jammed together are particularly sensitive to web blight. Warm to hot, humid weather patterns are essential for disease development. This disease only occurs on holly in landscapes during periods of unusually hot, wet weather.
Brown spots appear along the margin and base of the leaves. They rapidly enlarge into irregular brown to black blotches that may encompass the entire leaf. Mats of dead leaves usually cling to the damaged shoots. Blighting of the leaves usually starts on leaves closest to the ground and spread upwards through the canopy towards the shoot tips (Figure 2).
Figure 2. Web blight on Japanese holly cv 'Helleri'.
(Courtesy of A. Windham, University of Tennessee).
When possible, space plants to improve air circulation. Avoid overhead watering in late afternoon or early evening. Begin preventive fungicide sprays in June. Repeat sprays every 7 to 14 days as needed or until night temperatures cool. To get good disease control, thoroughly wet the canopy, stems, and media surface with the fungicide spray. Recommended fungicides are listed in Table 3.
Table 3. Chemical Control Of Web Blight.
Botryosphaeria or bot canker is a found on nearly all cultivated species of holly including 'Foster', dahoon, Japanese, Chinese, inkberry, American, and yaupon holly. Development of bot canker is often preceeded by exposure of holly to unusually high or low temperatures or in combination with severe drought conditions. Holly in good physical condition are rarely damaged by this disease.
The first noticeable symptoms of this disease are yellowing and premature leaf drop of the leaves on girdled limbs along with a twig dieback. Cankers, often centered on a dead twig, usually appear as slightly shrunken and cracked patches of discolored bark. Tissues below the canker face turn brown and this discoloration often extends several inches above and below the canker margin. Cankers may continue to expand until the diseased limb is girdled, thereby killing all parts of the plant above the canker.
Stress-related dieback diseases such as bot canker can largely be avoided by following proper establishment and maintenance practices. Preferably, new plantings of holly should be installed in the fall. Amend the soil prior to planting according to soil test recommendations to correct any mineral or pH deficiencies. Always set holly so that the root ball sits at or just slightly above the soil level, not below. On poorly drained soils or sites prone to flood, plant holly on raised beds. Mulch around hollies with well-rotted sawdust, pine straw, or an aged bark to hold moisture in the soil and to avoid mechanical injury to the limbs and trunk by mowers and weed trimmers. Water new and established plants according to need, especially during extended periods of hot, dry weather. Prune discolored or wilted branches back to green wood as they appear. No fungicides are recommended for the control of bot canker on holly.
Sphaeropsis knot occurs on holly primarily in Florida and to a lesser extent in other southeastern states. In Florida, this disease is found most often in landscape and native stands of American and dahoon hollies. Other hollies known to have been damaged by this disease in Florida landscapes and container nurseries include the Chinese holly, Japanese holly, yaupon, and 'Foster' holly.
Symptoms of Sphaeropsis knot range from localized swelling of young twigs to knobby galls on the larger branches. Also, clusters of stunted, sometimes leafless shoots or witches broom may be seen. Leaves on galled branches often turn yellow and drop. A dieback of the defoliated branches often follows.
Propagating cuttings from diseased holly is a common method of spreading this disease. Before taking any cuttings, inspect stock plants to make sure that they are free of symptoms. Do not take cuttings from diseased holly. After each cut, dip the pruning knives or shears in alcohol or a similar disinfectant. In the landscape, prune diseased limbs well below any galls or swellings. When establishing new plantings of susceptible hollies in Florida and nearby states, inspect incoming plants for typical symptoms and return any diseased hollies. No fungicides are currently labeled for the control of Sphaeropsis knot on holly.
In the landscape, provide effective protection from most leaf spot and blight diseases by
To minimize the length of time the foliage remains wet, water hollies in both landscapes and nurseries with overhead sprinklers between 1 and 7 am or at midday. If a serious leaf spot and blight should occur, collect and destroy the fallen leaves. Fungicides are rarely needed to protect holly in landscape plantings from leaf spot and blight diseases.
In container nurseries, dwarf Japanese and yaupon holly need to be spaced
out to allow air circulation and speed evaporation of water on leaf surfaces.
Fertility and watering practices detailed above will help control these
diseases in container-grown holly. Also, fungicides often must be applied
to protect rooted cuttings and container-grown Japanese and yaupon holly
through the spring and summer months from web blight. Recommended fungicides
are listed in the above tables.