FOCUS: Plant Diseases and Pests Four Fungal Plant Diseases for Spring A seasonal peek at the usual suspects

Four Fungal Diseases for Spring

by Heather Prince

With a mild winter and cool spring, fungal plant diseases are popping up in landscapes far and wide. As you assess client spaces, be on the lookout for these four fungal problems so you can be proactive in treatment. Fungal pathogens benefit from wet spring weather, and we chose two relatively new problems and two that regularly alarm clients every year. We consulted with university plant clinics in Wisconsin, Illinois, Michigan, and Indiana to provide a regional update on four problems that should be on your radar this spring.

Although it seems to make the news every year, only a smattering of boxwood blight cases occur throughout the Midwest. With the prevalence of boxwood in our landscapes, however, it’s a good idea to be able to recognize this devastating pathogen as immediate treatment is key to slowing and stopping its spread. Blight has been reported on cut boxwood greens used for Christmas decorations, so there is the potential for inadvertent introduction into the landscape. Boxwood have taken a number of hits the past couple of years, and it may also be a good idea to look at alternatives. The challenge is finding something with as much versatility and formal appeal.

Boxwood blight (Calonectria pseudonaviculata) represents first as dark leaf spots that may have a slight ring pattern that progress to twig blight and rapid defoliation. Look for dead spots in hedges and individual plants and inspect plant parts. Boxwood blight isn’t the only fungal pathogen that can cause twig dieback or dead patches, and it’s easy to classify winter damage, volutella blight, macrophoma leaf spot, or phytophthora root rot as blight. Clients may also lump all boxwood problems into ‘blight’. The big difference is that boxwood blight causes extensive defoliation and rapid death. The other pathogens listed cause foliage to brown or go tan, hold on the plant, and are far less likely to cause plants to die. They are easily pruned out and controlled.

Boxwood blight first presents as dark roundish splotches with yellow halos on leaves that may have white spores on the underside as the disease progresses. Rapid defoliation starts at the bottom and moves upward into the canopy. A key symptom that differentiates boxwood blight from other boxwood diseases is that narrow, black streaky cankers develop on green stems. During periods of high humidity,

white, fuzzy masses of numerous spores will develop from these black stem cankers and can be observed with a hand lens. This pathogen does not attack roots, but repeated defoliation and dieback from stem cankers can kill young plants quickly. If you suspect boxwood blight, send samples to your state plant clinic for confirmation.

Once the disease is detected, good sanitation is critical. Remove and bag diseased plants and fallen leaves and dispose or bury them. Where permitted, you may burn infected plants. Do not compost infected plants or plant debris as it will not kill the pathogen. Research has found that the fungus can persist in the soil for five years or more, which (continud on page 12)

(continued from page 10) means any replacement boxwood planted in the same site is likely to become infected. There are fungicides that are effective at protecting plants from boxwood blight infection, but they do not cure plants with the disease. Fungicides should be applied when temperatures exceed 60°F and rainfall is expected. Effective products include a rotation of chlorothalonil or fludioxonil. You will need to apply fungicides every seven to 14 days to protect susceptible boxwood. Growers are working on developing resistant boxwood varieties, and they are beginning to hit the market. Look for Little Missy and the NewGen™ series.

Sudden oak death (Phytophthora ramorum) was found in Wisconsin in 2019 on infected nursery stock of azalea. “There was only a single detection, but there were substantial numbers of potentially contaminated azalea plants and red ‘Double Knockout’ rose shrubs that were brought into the state and sold before the problem was discovered,” reported Brian Hudelson, of the Plant Disease Diagnostics Clinic at University of Wisconsin-Madison. It was first reported in California and has subsequently been found in other US states, as well as Europe. It can be found on a wide range of woody and herbaceous plants, but is fatal to oak trees.

Symptoms of sudden oak death vary greatly depending upon the plant species infected, but on oaks, look for dark brown leaf spots and blotches with fuzzy borders starting along leaf margins. The tricky part is these leaf symptoms may resemble more common leaf spots and blights or may mimic sunburn or leaf scorch. Twigs and branches that become infected often wilt and form a shepherd’s-crook at the tips, subsequently dying back. Infections on tree trunks lead to cankers that produce large amounts of amber to black ooze that can dry and stain bark. Remove the bark over the affected area and you’ll find discolored wood beneath that sometimes, but not always, has a black border. Cankers can eventually expand to girdle trunks, killing the tree or shrub. So far, trunk infections do not appear to extend into the root system. Once sudden oak death cankers develop, other pathogens may move in, accelerating tree or shrub death and complicating the diagnosis. Rapid wilting and die back of branch tips can be a symptom of ramorum dieback as well.

Phytophthora ramorum can be spread over long distances through movement of infected plants or infested plant parts. The pathogen can also be moved with contaminated soil or in contaminated water. Once established on plants, the fungal spores can be spread by rain splash or wind. There is currently no cure. Because Phytophthora ramorum is a regulated, quarantined pathogen, only remove a sample for diagnosis and do not remove the plant. Send the sample to your state plant clinic in a sealed container. Make sure to decontaminate any tools or materials that come in contact with the plant. If it is sudden oak death, the plant will be destroyed to prevent the infection from spreading. (continued on page 14)

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Anthracnose diseases are some of the most common problems of broadleaf shade trees in our area. This group of fungal diseases becomes severe when cool, wet spring weather persists as leaves are first emerging. The most commonly affected trees are ash, oak, maple, and sycamore, although you may find it on dogwood, birch, elm, walnut, butternut, hickory, and others depending on the year. This is a host-specific disease, so it won’t pass between trees; each tree species has its own fungus species.

Symptoms of anthracnose diseases are irregular leaf spots and blotches most often near the veins of a leaf causing it to curl and distort. Leaves may die and drop, causing client alarm. Fungi that survive in leaf litter will have spores carried up with spring rains and wind, so damage is usually found on the lower branches of the tree. If you see curled or twisted leaves at canopy tips or on exposed edges, it may be frost damage instead. In some tree species, especially sycamore, the pathogen can cause branch cankers, causing twigs to die back or produce distinctive witch’s brooms. The good news is that this disease does not usually cause tree death. Vigorous trees that lose their first leaves usually will push out another set, so it is important to guard tree health. Fungicide treatments are not recommended as by the time you spot damage, it is too late to treat and it is common on large shade trees, making

coverage by fungicides difficult. For high-value trees of smaller scale, however, fungicides may be applied starting at bud break the following year. Fungicides listed for anthracnose include chlorothalonil, mancozeb, propiconazole, and copper-based formulations. (continued on page 16)

(continued from page 14) Rhizosphaera needle cast (Rhizosphaera kalkhoffii) is one of the most common fungal diseases of Colorado or blue spruce (Picea pungens). It can also be found on other spruce trees, but blue spruce seems to be among the hardest hit, especially mature trees. Wet spring weather the past few years has accelerated this disease and made it far more prevalent. You’ll first notice browning on the innermost needles on the lowest branches, with the tips of twigs remaining healthy. In the early stages, affected needles may turn purple or lavender before browning. When examined with a hand lens, rows of black spheres about the size of a grain of pepper run the length of the needles. These are the fruiting bodies. Eventually, infected needles fall to the ground. Fallen needles are a good vector for repeat infection as spores are bounced back into the tree by rain. Cleaning up fallen needles will help limit the spread.

Rhizosphaera can be treated with copper or chlorothalonil fungicides, although they will act as a preventative for further infections, not a cure. Apply every three to four weeks starting when new needles emerge in spring and continue into summer and drier weather. Treatment is best done on small or specimen trees as large mature spruce are difficult to cover effectively. If using spruce in the landscape, make sure they have plenty of air movement, good drainage, and full sun to maintain tree health.

These are just a few of the plant diseases moving through the landscape

in spring. The university extension services are here to help as well as The Morton Arboretum and Chicago Botanic Garden. By paying attention and scouting, not only can you save valuable landscape plants, you may also be able to add another line item to your invoices.