Lilac Blights

A long rainy spring, such as many of us experienced this year, creates the perfect conditions for lilac blight. There are two types of lilac blights, a fungal variety, Ascochyta syringae, and a bacterial variety, Pseudomonas syringae p.v. syringae (the p.v. stands for pathogen variety and indicates a subspecies). Both varieties affect new growth on lilacs, and, since the symptoms can appear somewhat similar, we’ll look at each blight separately.

Ascochyta syringae

Ascochyta is a disease that usually affects grains and grasses; it causes large, wheat-colored  patches of dead grass on bluegrass, for example. The fungus can also appear as leaf spots on various trees, including ash, boxelder, dogwood, and walnut, among others. Before being renamed Phoma clematidina, Ascochyta clematidina was the name of the pathogen responsible for clematis wilt. So Ascochyta is a well-distributed plant pest.

On lilac, Ascochyta is sufficiently severe to cause a blight, in which the leaf spots coalesce into blotches. Eventually the blight girdles new twigs and kills the leaves. In the spring, look for symptoms on new growth, including a brown or black shepherd’s crook. This is one of the confusing symptoms, since shepherd’s crooks are more commonly seen with bacterial diseases, such as fireblight on crabapples.

Another symptom of the fungus is leaf spots. Generally, Ascochyta will have a tan center with a different colored margin of decaying tissue—typically a darker brown—with indistinct, rounded edges. Bacterial leaf spots are usually more angular in structure.

Fungal diseases are spread by splashing water, wind, insects, gardening tools, and touching affected plants while they are wet. Natural disease control methods include proper plant spacing and pruning to allow rapid drying of foliage, pruning out infected branches (use an antibacterial spray on secateurs between cuts), avoiding high nitrogen fertilizers, controlling powdery mildew, raking up and disposing of all dead leaves, and avoiding overhead watering. There are also systemic fungicides that can help bring Ascochyta syringae under control while pursuing the non-chemical approaches to improved lilac health. As with any chemical control, always read and carefully follow instructions on the label.

Pseudomonas syringae p.v. syringae

Bacterial blight on lilac

Bacterial blight on lilac

The more commonly seen lilac blight is the bacterial blight Pseudomonas syringae p.v. syringae. Symptoms of bacterial blight also include blotchy brown patches on leaves, but the edges of the damage will appear more angular and may be surrounded by a thin, yellow halo. Twigs will also appear dark brown and dead, similar to the symptoms of fireblight, but, instead of forming a shepherd’s crook, the twig appears broken.

Windy, rainy weather just prior to flowering favors development of Pseudomonas. The disease can be spread by splashing water, wind-driven rains, insects, birds, humans handling plants, and tools which haven’t been disinfected. Unlike many fungi, bacteria reproduce most effectively as parasites of the plant host rather than in plant litter or in the soil.

The best method of control is to plant lilac varieties that are resistant to bacterial blight. Dr. Leonard Perry, a noted horticulturist at the University of Vermont (I recommend his excellent Perry’s Perennial Pages for plant specific information) has put together an extensive list of lilac trials in Vermont for susceptibility to blight. Although the common wisdom with lilacs is that white-flowered varieties are more prone to blight, Professor Perry’s list contains several white-flowered varieties with minimal bacterial blight issues.

If you already have a favorite lilac that you want to keep blight-free, follow the same cultural procedures for lilac health mentioned under Ascochyta Blight. To control existing bacterial blight, use an antibacterial spray on your secateurs and cut away the affected twig at least 6” (15 cm) below the damaged area. Repeat spraying the secateurs between each cut. For severe problems, or if you live in an area with especially wet spring weather, you may wish to purchase a fungicide with copper sulfate, using it before new growth begins each year, up to flowering, and after flowering until new growth ceases. Carefully follow all label directions.

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Alternaria Leaf Spot Fungus on Aster ‘Purple Dome’

Regrettably, I can no longer recommend Aster (syn. Symphyotrichum) novae-angliae ‘Purple Dome’ due to its susceptibility to Alternaria leaf spot. Alternaria is a fungal leaf spot disease that can affect trees, as well as perennials. Although often not serious on trees, it can be particularly damaging on perennials because the fungus is saprophytic—meaning it perpetuates itself on dead plant tissue and in the soil.

Alternaria leaf spot on Aster 'Purple Dome'

Alternaria leaf spot on Aster ‘Purple Dome’

The damage begins closest to the ground and works its way up the stem. The lowest leaves turn brown and feel crispy, while leaves just above turn yellow and spotted, as shown in the photo at left. Eventually the yellow, spotted leaves will also turn brown and die.

The fungus is spread by splashing water and is easily spread by normal rain showers. Due to the compact nature of ‘Purple Dome’s’ growth, it’s impossible to thin out the foliage to increase air flow without noticeably affecting the plant’s appearance. Additionally, the disease stunts the plant’s growth so that it never achieves its potential spread and height.

In order to control the disease, it is necessary to constantly remove and permanently dispose of dead plant tissue throughout the summer and fall. Even then, saprophytic fungal spores may still remain in the soil. Additionally, a systemic fungicide needs to be applied in the spring, as leaves first emerge, and then regularly throughout the summer. Although I don’t have a problem with using fungicides to bring a disease under control, any plant that requires constant chemical sprays throughout its growing season is not a superior selection. If you choose to grow ‘Purple Dome’, be aware of the time and cost involved to maintain the plant’s health and appearance.

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Plant of the Month: Rudbeckia hirta ‘Indian Summer’

Rudbeckia hirta 'Indian Summer'

Rudbeckia hirta ‘Indian Summer’

Rudbeckia hirta ‘Indian Summer’ is a superior breeding improvement on the classic Black-Eyed Susan. Like it’s parent, Rudbeckia hirta, ‘Indian Summer’ has hairy leaves and stems, the foliage is alternate and spread out along the main stem, there is only one flower per plant, and both are biennials. There the comparison ends. Continue reading

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Oystershell Scale on Lilac

My neighbor’s lilacs, while showing signs of lilac blight earlier in the season, continue to experience massive dieback. Today, I decided to perform a closer examination, since the proximity of his plants puts my plants at risk.

Lilac dieback from oystershell scale

Lilac dieback from oystershell scale

To my surprise, his lilacs are covered with oystershell scale, and the infestation is so severe that it isn’t worth trying to save the shrubs. Certain types of scale, such as euonymous scale, are ubiquitous, but oystershell scale–especially when it has clearly been present for several years–is unusual among reasonably well tended suburban gardens. My guess is that he probably purchased inferior stock at one of the local big box stores. Combined with yearly bouts of powdery mildew, and a location with insufficient airflow, the stressed shrubs were doomed to an attack by opportunistic insects. Continue reading

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Invasive Species

Lythrum salicaria in a colorful border

Lythrum salicaria in a colorful border

Back in the 1980s, it was almost impossible to pick up a gardening book or horticultural publication that didn’t recommend planting species that are banned or restricted today as invasive plants. Some states have extensive lists of banned or controlled species, while other state lists are more moderate. Sometimes the lists make no sense unless you understand the particular economics of a state’s agriculture. The bans or restrictions may be specifically to protect grazing cattle or cranberry bogs or even wildlife restoration. More often, the listed plants are so aggressive that native species are being crowded out. Continue reading

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Eye Candy for Gardeners

Rosa Morden Snowbeauty

Rosa Morden Snowbeauty

This photo seems to perfectly capture one reason gardeners have such a long-standing love affair with the rose. I can’t think of another flower as delicate and elegant when first opening as a rose. The Morden Snowbeauty rose shows just the faintest hint of pink before opening to pure white. Aren’t roses fantastic?

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Bristly Roseslug

Bristly Roseslug leaf damage

Bristly Roseslug leaf damage

It never ceases to amaze me how all it takes is one rose in a garden in order for the Bristly Roseslug to find it and attack. The roseslug is the transparent (when young) or green (when older) larval caterpillar of a sawfly (wasp), Endelomyia aethiops. The larvae are no more than one-half inch in length and, if you’re lucky, can be found  while eating leaves and promptly squished. Younger larvae skeletonize the leaves, as seen in the photo at left, while older larvae chew holes in the leaves, often along the leaf edges. Continue reading

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