July Diagnostic Lab Preview
Problems we usually see this month
Normally, July is a big month for samples with spider mites, leaf scorch, and plants stressed from under- watering. In some years we have had multiple wet spells when our area received a lot of rain over several days. In these years there is less drought stress than usual, but perhaps more root rot. Of the other problems we have discussed in previous months, we will still see samples of anthracnose on trees, early and late blight on tomatoes, lacebugs, various fungal leaf spots, and scale insects. Many of these problems began in spring but are only now being noticed by homeowners.
There are too many new problems arriving this month to cover them all here. For the problems we have seen less often, we are only providing a few notes and references at the end of this document.
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 Pest Management Guide for pesticide recommendations.
There are several hundred different species that cause downy mildew diseases on plants. They are fungus-like organisms called oomycetes (water molds), in the family Peronosporaceae. Most downy mildew species are specific to a single plant species, genus, or family. In recent years, we have seen downy mildews on cucumbers, basil, grape, impatiens, and rudbeckia.
The disease has a different appearance on different hosts. It can cause blighted or distorted leaves, but it often appears initially as discolored spots on upper leaf surfaces, usually off-color green, yellow, tan, brown or black. Spots are usually angular, bounded by leaf veins, but expand and coalesce as the disease progresses. The key to diagnosing this disease is on the undersides of the leaves. Beneath the discolored spots, there will be fuzzy (“downy”) growth of spore-producing structures that can be white, gray or nearly black.
On cucurbits, spots are yellow, turning tan then brown as the leaf is killed. On undersides, spots initially appear watersoaked, then are covered with dark purplish-gray sporulation.
On basil, you see puckered or distorted leaf shapes. Discoloration begins as yellow- green areas that turn dark blackish green. Sporulation will be a warm gray.
On impatiens, symptoms may not be very obvious on upper leaf surfaces – yellowing or yellow stippling, downward cupping of leaves, and a general lack of vigor. Sporulation is white.
The available chemical controls only protect treated plants against infection, do not eradicate the disease, and must be frequently re-applied. The general recommendation for annual plants is to dig out any infected plants, seal them in plastic bags, and dispose with household waste. (Do not compost and do not recycle with yard waste.)
Because the downy mildew organisms are not true fungi, many of the traditional fungicides used to control powdery mildew or other fungal diseases are not effective against the downy mildews. Even products labeled for downy mildew may be effective only on some downy mildew species and give little control on others.
Under Home Vegetables, the Pest Management Guide’s Disease Management Tools for Specific Crops and Diseases are found in Table 2.4. Recommendations for ornamentals are found in Table 4.1 under Oomyceteous diseases, Downy mildew (page 4-7). Downy mildew on grapes is discussed on page 3-17. Control recommendations are also found in the Spray Schedule for grapes (Table 3.8).
Management of Cucurbit Downy Mildew for Gardeners, Michigan State University Extension
Eriophyid Mite Galls
There are many different species of mites in the family Eriophydae that cause deformities in leaf, bud and twig growth. Some cause blisters, some rust spots, and others witches broom, but the damage we have been seeing most are the leaf and bud galls. Our recent samples have included black gum, caryopteris (or bluebeard), and maple.
These microscopic mites infect leaves and buds of plants in early spring. They continue to feed during the growing season, injecting a chemical that causes the abnormal growth. Galls will have a tiny exit hole on the underside. They can be cushion shaped (like the Maple Bladder Gall in the photo) or elongated (sometimes referred to as finger or spindle shaped). Galls often turn from green to yellow or red as summer progresses.
On caryopteris, the mite attacks leaf and flower buds, forming warty-looking galls around the stems where the flowers should have been. Galls start out green and turn brown by mid-summer. If not removed, galls can persist for several years.
Leaf and stem galls can also be caused in different plants by small insects, including tiny wasps, midges, adelgids, aphids and psyllids. Correctly identifying the cause usually begins by identifying the plant host and researching its known pests.
While galls may be cosmetically undesirable, rarely is enough foliage affected to cause trees serious harm.
For bud galls on caryopteris, prune out infected stems once plants are dormant to reduce overwintering populations. As this plant blooms on new wood, stems can be cut to the ground in winter without affecting next year’s blooms. Apply controls in spring as buds swell.
The Pest Management Guide control recommendations for eriophyid mites are found in Table 4.5 under Mites. Treatment needs to be applied in early spring before new growth develops. By the time galls are noticed, it is usually too late to apply controls that year.
Insect and Mite Galls, Colorado State University Extension, Fact Sheet 5-557
Eriophyid Mites, Missoula County Extension Office Plant Diagnostic database
Mite galls on Caryopteris, University of Arkansas Plant Health Clinic News
Fire Blight on Pears
Fire Blight is caused by the bacterium Erwinia amylovora. It attacks only plants in the Rose family, including apple, pear, quince, pyracantha, cotoneaster, mountain ash, hawthorn, and raspberry. All our recent samples with this disease have been pears.
The disease usually begins in the spring with infection of blossoms and new shoot growth. Leaves initially turn a dark green or gray green, then rapidly change to brown then black. Blackened twigs look as though burnt in a fire. The infection moves down the twigs and into branches. If the infected branches are not removed, the plant will eventually die.
Some keys to diagnosing this disease are:
- Leaves and twig stems turn black.
- Affected twigs and fruit spurs often curl near the end, giving a “shepherd’s crook” appearance.
- Freshly blighted tissue may exude amber-colored droplets containing bacterial cells.
- Stem cankers are darkened and slightly depressed, sometimes bordered by a narrow light brown or tan color callus ridge.
- When bark is scraped off, inner wood of infected twigs has a reddish-brown discoloration.
Some fungal diseases (such as brown rot on cherries) can cause similar looking damage. If fungal fruiting bodies are present on diseased (but still living) tissue, the stem dieback likely had a fungal cause.
Avoid heavy nitrogen fertilization, especially in summer when succulent growth is most susceptible to fire blight infection. Prune out cankered limbs and branches 8 inches below the damage. Avoid pruning when the plants are wet. Dip pruning tools in a sterilizing solution of 70 percent isopropyl alcohol or 10 percent chlorine bleach between each cut. Prune out water sprouts and suckers, which are highly susceptible to infection. Fire blight control is difficult and expensive. Chemical control is not always effective and needs to be applied preventatively. There are apple and pear varieties with some resistance to this disease.
The Pest Management Guide general chemical recommendations for control of fire blight on ornamentals are found in Table 4.1 under Bacterial, Fire blight (page 4-10) and more specifically in Table 4.2 for Crabapple and Ornamental Pear.
Fire blight on apples is discussed on page 3-9. Control recommendations are also found in the Spray Schedules for apples (Table 3.5) and pears (Table 3.6).
Fire blight – Trees, University of Maryland, Home & Garden Information Center
Fire Blight – Plant Diseases, Penn State Extension
Fire Blight, University of Illinois Extension Hortanswers
Japanese beetles begin hatching in late June and will feed and lay eggs until early August. They are easily recognized by their bottle green color and coppery wings. Beetles feed on a wide range of woody and herbaceous plants, including leaves, flowers, and over ripe fruit.
Beetles will sometimes feed on leaf margins, but their damage is more commonly many small holes between the veins, sometimes completely skeletonizing the leaves. In heavy infestations, leaves may be eaten all the way to the midrib. If you see only scattered, tiny holes, the culprit is more likely a flea beetle.
Although the damage is not attractive, plants will not generally die from beetle leaf damage. As flying insects, they are difficult to control with sprays, and tend to be present in similar numbers whether or not controls are attempted.
Hand removal can be effective when beetle numbers are low. Early in the morning, when insects are sluggish, shake beetles off branches into a container of soapy water.
The Pest Management Guide recommendations for control of this insect are found in several places.
- Control recommendations are found in Table 2.2.30 Insecticides.
- Control recommendations for fruit are found in Section 3 in the individual spray schedules.
- Control recommendations for ornamentals are found in Table 4.5 under Defoliators, Japanese Beetles and in Tables 4.7 and 4.8.
- Control recommendations for Japanese beetle grubs in turf are found in Table 5.2 under White Grubs.
Leafhopper is a common name applied to any species from the family Cicadellidae. They are tiny (about 1/8″ long) and slender. You are unlikely to see the insect itself on samples brought to clinics, as adults are winged and hop off the plant when disturbed. Nymphs cannot fly, but move quickly.
Leafhoppers feed on a wide range of plants by piercing leaf cells and sucking out the contents, causing the leaf tissue to turn white. This stippling can look similar to lacebug or spider mite damage. Examine the undersides of leaves to eliminate those possibilities.
On most plants, damage is limited to stippling on leaves and sometimes leaf curling. On potatoes, leafhoppers cause “hopper burn” – yellowing and downward curling of leaves and brown leaf tips and margins. Leafhoppers are also known vectors of plant virus and bacterial diseases.
The Pest Management Guide recommendations
for control of leafhoppers are found in several places.
- Control recommendations are found in Table 2.2.16 Insecticides.
- Control recommendations are found in Table 3.8 in the Grape spray schedules.
- Control recommendations for ornamentals are found in Tables 4.5 and 4.6 and in Table 4.8, Organic Controls.
Phyllosticta Leaf Spot
Several members of the fungal group Phyllosticta sp. cause leaf spots on a variety of woody and herbaceous plants. Locally, Phyllosticta minima infect many maple tree species. Lesions begin as brown spots on leaves that enlarge with centers becoming tan in color. Edges of the lesions have purple or red rims, hence the other common name, purple-eye leaf spot. Lesions can merge and centers fall out creating ragged holes in the leaf. Some leaf drop may take place when there is extensive infection. However, new leaves grow in summer to replace those lost in spring.
Phyllosticta on leucothoe
Phyllosticta on maple
These infections are usually more cosmetic than threatening to healthy hosts and fungicides are not generally recommended. Instead, clients should be advised to
- focus on good cultural practices, including watering deeply (1 inch/week) during dry periods and mulching to provide moisture retention, especially on young plants
- practice good sanitation procedures by raking and bagging fallen leaves for trash disposal
- prune (with sharp, sterile tools) to increase air circulation and to remove worst infected areas
There are 5000 or more species of fungi that cause diseases we refer to collectively as rust. They infect crops, ornamentals, turfgrass and other plants. Most rusts are host specific.
Rust pustules on Hollyhock
Rust typically appears as small, bright orange, yellow, or brown pustules on the underside of leaves. If touched, they leave a colored spot of spores on the finger. There is typically a corresponding off-color spot (often yellow) on the upper leaf surface. Like many fungal diseases, rust is worse when weather is moist.
In recent years, we have seen rust on hollyhock, beans, azalea, daylily, and hydrangea. It is also sometimes seen in our area on iris and bluegrass.
The Pest Management Guide recommendations for control of rust are found in several places.
- Control recommendations for vegetables are found in Table 2.4 under the individual vegetables.
- Control recommendations for ornamentals are found in Table 4.1 under Fungal Diseases, Rust.
- Control recommendations for Turf are found in Table 5.1.
Shot Hole Disease on Cherry Laurel
Shot hole disease causes holes in leaves of various sizes and sometimes leaf margins that appear ‘scooped out’. This disease is caused by a combination of the bacteria Xanthomonas arboricol pv. pruni and/or Pseudomonas syringae and several fungal pathogens, including Cercospora species. Both fungal and bacterial pathogens can be present on the plant at the same time. These pathogens can infect plants in the genus Prunus, but all our samples have been Cherry laurels (Prunus lauroserasus). The disease begins as circular leaf spots, usually red, brown or wet looking. Centers turn tan before dropping out. These holes and notched edges can easily be mistaken for chewing insect damage. When spots have a reddish edge and a yellow halo, the bacterium is present. You may also see stem cankers. Symptoms will look somewhat different on deciduous Prunus species.
Prolonged wet, warm conditions from May to September contribute to the development and growth of these infections. Most of these pathogens can overwinter on the plant and in leaf litter around the plant.
Ordinarily, shot hole disease does not threaten the life of cherry laurels and is usually considered a cosmetic disease that does not need chemical treatment. Infection can be limited by following the same basic cultural advice given for many fungal diseases.
- Provide adequate space for plants to mature with good air circulation
- Avoid overhead sprinklers/watering<
- Collect, bag and dispose of fallen, infected leaves to minimize re-infection and reduce overwintering innoculum
- Prune out worst infected branches when conditions are dry with sharp, sterile tools
The Pest Management Guide does not provide any chemical control recommendations for bacterial leaf spot or shothole disease in ornamentals. When a fungal organism is also present, some control may be obtained by use of a commercial fungicide labeled for the plant.
Other Problems Worthy of Mention
- Hosts are arborvitae, spruce & others
- Caterpillars strip needles/leaves which they eat and use to build cone- shaped bags around themselves. Eggs overwinter inside the bag. If possible, remove bags from plant. Where plant is too large or infestation too extensive, apply controls next June when caterpillars are small.
- Bagworms, VA Tech Fact Sheet 2808-1008
- on azalea, camellia, dogwood
- Interveinal chlorosis is usually a nutritional problem related to soil pH, common on plants needing acid soils. Recommend a soil test.
- Chlorosis, University of Illinois HortAnswers
English boxwood decline
- English boxwood
- Decline of mature boxwoods has multiple causes and often follows drought stress. There are no recommended treatments. As soil fungi are contributing factors, English boxwood should not be replanted in the same location where one has died.
- Boxwood Blight, Virginia Cooperative Extension
Phytophthora root and stem blight
- on annual vinca(Catharansus rosea)
- If the entire plant is wilting severely, it has root rot and cannot likely be saved. If only individual stems are affected, they should be cut out at the base and the remainder of the plant may continue to grow and recover.
- Vinca Diseases, Clemson Cooperative Extension
Phytophthora stem dieback
- Stem dieback begins as leaf spots on new foliage. The disease moves down the stem causing leaves to brown. Prune out diseased branches below affected tissue, until the cut stem is white. Can look similar to Botryosphaeria dieback. A V-shape pattern of browning along the leaf midrib, starting at the petiole, indicates Phythophthora.
- Phytophthora Dieback and Root Rot, University of Connecticut IPM
- on arborvitae, leyland cypress
- This very tiny (2 mm) scale is round, nearly white, and found on undersides of needles of juniper, arborvitae, and cypress. Off-color foliage is usually the first symptom noticed. Can cause stem dieback and death of the plant if untreated.
- Juniper Scale, Washington State University
- holly & others
- Mold is most likely feeding on honeydew secreted by a sucking insect pest and will usually stop once the insect problem is corrected.
- Sooty Molds, North Carolina Extension
Fusarium wilt and Verticilium wilt
- These vascular diseases are caused by fungi entering the plant through its roots. Both organisms remain in the soil for years. There are no chemical controls for either. Recommend selection of resistant varieties, sanitizing digging tools, and planting in a different location.
- Fusarium Wilt of Tomato, University of Maryland Extension
- Verticillium Wilt of Tomato, Cornell University Cooperative Extension
- tomato, eggplant, hydrangea
- Tiny, white flying insects suck juices from plants and can cause sooty mold and loss of plant vigor if infestation is severe.
- Controlling Whiteflies in your Garden, University of California VC Master Gardeners page
- Managing Whiteflies on Indoor and Outdoor Plants, University of Missouri Extension
Prepared by: Susan McCullough
Fairfax County Master Gardeners Association sponsored by Virginia Cooperative Extension
Release Date: May 2013
Links updated: July 2020