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 2018 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.
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.
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.
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. This is a soft scale, producing quantities of honeydew, so sooty mold often accompanies this pest.
This scale has one generation per year. In early spring, the female moves to the underside of a leaf and begins laying eggs, which is when the pest is often first noticed by homeowners. We have seen samples with this pest May through August.
This scale overwinters in its immature stage and is well controlled by horticultural oil sprays in the dormant season. Other controls should be applied June 15-30. Severely infested twigs can be cut out.
The Pest Management Guide pesticide recommendations for control of this insect are found in Table 4.5 under Scale, General.
Field Key to Identification of Scale Insects on Holly, University of Florida IFAS Extension IPM-141
Cottony Camellia Scale – Shrubs, University of Maryland HGIC Landscape Problem Solver
Septoria Leaf Spot on Rudbeckia
Septoria fungi cause leaf spots on a variety of hosts, but the one that infects Rudbeckia is Septoria rudbeckiae. The disease first appears as small brown spots that expand and coalesce to form large brown areas. Spots take on an angular look as their spread is stopped or slowed at leaf veins. The disease begins on lower leaves. Black fungal fruiting bodies can be seen in the center of older spots.
Mild cases of the disease are mostly cosmetic, as diseased plants will still bloom. When the disease appears early in the season, it can severely defoliate plants as the infection spreads to upper leaves.
In the fall, cut plants down and remove all infected materials from the area to minimize overwintering inoculum. During the growing season, avoid wetting the foliage and improve air circulation by keeping plants well-spaced.
Chemical controls serve as protectants, preventing spread of an infection rather than curing it. When warranted, fungicide applications should begin in late May or early June. The Pest Management Guide chemical recommendations for control of this disease are found in Table 4.1 under Septoria leaf spot.
Thrips are rasping-sucking insects that feed on growing tips and flower buds, causing discolored and distorted leaves, flower petals that are discolored or scarred, and buds that fail to open. Feeding damage can cause browned areas or silvery white patches.
There are several species of thrips in our area. They can be tan, black or very pale yellow-green. Most are about the size of a fine print hyphen (less than 1/16″). Because they usually feed deep inside the flower bud, they are often not noticed until the damage is apparent.
Since thrips feed in areas that are difficult to reach with sprays, multiple applications of insecticides are often necessary for adequate management.
- Control recommendations for thrips on vegetables are found in the Organic Controls Table 2.1 and in Table 2.2.8 Thrips.
- Control recommendations for thrips on fruit are found in Section 3 in the individual spray schedules.
- Control of thrips on houseplants is discussed on pages 4-66 — 4-69.
White Peach Scale on Cherry Laurels
White Peach Scale (Pseudaulacaspis pentagona) is a common pest of both fruit trees and ornamental plants in the genus Prunus, along with lilac, privet, walnut and persimmon. In this area, we also get White Prunicola Scale (Psuedaulacaspis prunicola) which looks so similar they used to be regarded as the same species. Sometimes, the plant has both. It is not necessary to distinguish between them at Plant Clinics, as the same controls are effective on both.
Infestations look like a pale waxy crust on twigs or stems. The cast skins of males sometimes give the stem a fluffy appearance. Foliage of infected plants may look off color. In this area, there are three generations a year. We have received cherry laurel samples with this pest May through October.
Female white peach scales appear round while males are slender. Females under their wax armor look like fried eggs, shown here with crawlers.
Scrub infested bark with a soft brush dipped in mild soap solution or by power washing. Prune out severely infested branches if practical.
Horticultural oil sprays in the dormant season (late March-early April) are effective on this pest. Contact insecticides are ineffective once these scales have built their waxy coating, so timing of sprays when crawlers are active is essential. For white peach scale, the PMG advises applying crawler-stage controls May 1-10; July 5-15; and September 1-10. Crawlers are out about 2 weeks earlier for white prunicola scale.
The Pest Management Guide pesticide recommendations for control of White Peach Scale on ornamentals are found in Table 4.5 under Scale, White peach scale. For scale on Prunus fruit trees, refer to Table 3.7.
All pesticides recommended are at the risk of the user. Read and follow all label instructions for use, storage and disposal; the pesticide label is the law. Please refer to Fact Sheet 426-710, Applying Pesticides Safely.
Prepared by: Susan McCullough
Fairfax County Master Gardeners Association sponsored by Virginia Cooperative Extension
Release Date: May 2013
Links updated: May 2018