May Diagnostic Lab Preview
Problems we begin to see this month
May marks the beginning of our plant clinic season. During this month we will already be seeing a variety of insect, disease and abiotic problems. In addition to the perpetual suspects (aphids, spider mites, slugs, etc.) here are some of the things we regularly see in May samples.
These reports are prepared for use at plant clinics by Master Gardeners. They are republished here to assist experienced gardeners in identifying and treating plant problems. Please remember to refer only to the Virginia Cooperative Extension 2021 Pest Management Guide for pesticide recommendations.
Phomopsis Tip Blight
Caused by the fungus Phomopsis juniperovora, this disease kills new growth on conifers in the cypress family (including Juniper, Arborvitae, Cedar, Falsecypress). Older twigs are resistant to infection. Also called Phomopsis needle/twig blight, this disease occurs during prolonged wet, cool periods, usually in spring or fall. Spores overwinter on infected stems and are spread by rain, overhead irrigation, or people to other branches and adjacent healthy plants.
The disease is first noticed as new growth turns paler, then reddish and then brown. On examination, you will see a grayish area at the base of the diseased tissue. In severe cases, the disease will move down the twig into the stem.
Prune out and destroy diseased tips as they appear. Cut four to six inches below diseased areas on each branch. Disinfect pruning shears after each cut by dipping them for at least 30 seconds in a solution of 10% chlorine bleach or 70% alcohol. Prune only when foliage is dry.
The Pest Management Guide chemical recommendations for control of this disease are found in Table 4.1 under Phomopsis needle blights.
Juniper Tip Blights, Virginia Cooperative Extension Publication 450-601W
Phomopsis Tip Blight of Juniper, Ohio State University
Exobasidium Galls on Azalea
This leaf and flower gall, caused by the fungus Exobasidium vaccinii, is a fairly common fungal disease of Azalea, occurring during spring when the weather is wet and mild. Its severity varies from year to year based on weather conditions. Exobasidium galls caused by related species also occur on lowbush blueberry, bog cranberry, and camellias.
The gall coats the leaf or petal, causing it to curl and cup into a gall shape. It starts out green, then develops a fuzzy white coating. As the gall ages, it turns tan to dark brown.
As soon as the disease is noticed, diseased flowers and leaves should be hand-picked, placed directly into a plastic bag, and removed from the area. Dispose with household waste. By the time the disease appears, it is too late to apply fungicides this year. Chemical controls are not usually necessary in the home landscape. If this has been a recurring problem or where the problem is severe, fungicides can be used next spring to minimize recurrence of the disease. The fungicide is applied just before bud break as a preventative.
The Pest Management Guide chemical recommendations for control of this disease are found in Table 4.1 under Leaf/flower gall.
Azalea Leaf and Flower Gall, Virginia Cooperative Extension Publication 450-605
Camellia Leaf Gall, Clemson Cooperative Extension video
Entomosporium Leaf Spot on Red Tip Photinia
Caused by the fungus Entomosporium maculatum, this is a common disease of red tip photinia, Photinia x fraseri, and other members of the rose family.
The disease first appears as small bright red spots on both surfaces of new leaves. They grow into large purple to maroon spots and blotches usually with red halos. On older spots, dead centers turn tan and black pycnidia can often be seen. Spots can also develop on leaf petioles and tender stem growth. While light infections usually cause little more than cosmetic damage, severe infections often result in early and heavy leaf drop.
The primary sources of disease are spores overwintering on infected stems and leaves. The disease is spread by water splashing from leaf to leaf by rain or irrigation. This fungus is encouraged by cool weather and poor air circulation.
Cultural practices can reduce the severity of the disease. Do not water or fertilize plants any more than necessary to avoid promoting excess new growth. If it is necessary to irrigate the plants, do not wet the foliage. Avoid pruning during the summer which promotes continual new growth. Remove fallen diseased leaves. Next winter, prune to thin out plants and promote air circulation.
Because severe defoliation can be devastating on this plant, chemical controls are sometimes warranted. Multiple applications are generally required for season-long protection. See product label for repeat intervals. The Pest Management Guide chemical recommendations for control of this disease are found in Table 4.1 under Entomosporium leaf spot.
Entomosporium Leaf Spot of Photinia, Virginia Cooperative Extension Publication 450-609
Photinia (Red Tip), Clemson Cooperative Extension HGIC 1081
Cottony Camellia Scale on Holly
Cottony Camellia Scale (Pulvinaria floccifera) feeds on camellia, holly, yew, euonymus and maple, and is sometimes also found on beautyberry, jasmine, mulberry, English ivy, hydrangea, and sweet box (Sarcococca). When found on yews, this same insect is called Cottony Taxus Scale.
The adult scale insect has a shiny brown or brown and yellow covering; crawlers are yellow-green. It is the egg masses that are cottony. Th