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. Continue reading

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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. Continue reading

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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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