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Look out for disease problems on new plants

Michelle Grabowski, University of Minnesota Extension

I always recommend that gardeners thoroughly inspect every plant for disease problems before they purchase it. Examine both the upper and lower surfaces of the leaves, especially those leaves closest to the soil. Look at the stems and even the roots. Any spots, streaks, dark discolored or soft, mushy tissue is a warning sign that your new plant may come with a disease problem. Once a new pathogen is introduced into a garden, it may cause problems for years to come. In addition, nursery plants may move across the country before they make it to a local garden center. As a result infected plants can bring new plant diseases to Minnesota.

Boxwood Blight

Virginia Tech, Plant Disease Clinic

Photo 1: Leaf spots and blighting due to boxwood blight

One disease of concern is boxwood blight, a fungal disease caused by Cylindrocladium buxicola (syn C. pseudonaviculatum). Boxwood blight was first identified in the eastern United States in 2011. It has since been found in 8 eastern states as well as in Oregon. To date boxwood blight is known to infect all species of boxwood (Buxus spp.) and Pachysandra sp.

Boxwoods infected with the boxwood blight fungus first develop tan leaf spots with a dark brown border. Chalky white spores form on the lower surface of the leaf just below leaf spots, under humid conditions. Leaf spots grow large enough to blight the entire leaf. Infected leaves fall off, resulting in bare branches throughout the shrub. The fungus also infects branches anywhere from the soil line to the tip of the branch. These infections start out as dark brown to black short lines, and grow to completely encircle and girdle the stem. Roots of the boxwood shrub are not infected by the fungus and remain healthy.

Virginia Tech Plant Disease Clinic

Photo 2: Stem lesions caused by boxwood blight

Boxwood blight has not yet been identified in Minnesota. If you suspect your plant is infected with boxwood blight first use the University of Minnesota Extension's online diagnostic tool for Boxwood to determine if the problem might be a more common look alike. If Boxwood blight appears to be the most likely suspect, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

Downy Mildew
A second invasive disease to be on the look out for this year is downy mildew of basil. This disease is caused by the fungus-like organism Peronospora belbahrii. Downy mildew on basil was first found in the United States in Florida in 2007. It was seen throughout the eastern states in 2008 and was identified in Wisconsin in 2010. Downy mildew of basil has not been officially identified in Minnesota.

M. McGrath, Cornell University,

Photo 1: Downy mildew symptoms on the upper leaf surface of a sweet basil plant

Only basil plants (Ocimum sp.) are susceptible to this species of downy mildew, although other species of downy mildew can infect other common garden plants. The commonly grown sweet basil, Ocimum basilicum, is highly susceptible and growers have reported 100% crop loss due to the pathogen. Infected plants first display yellowing on the lower leaves. As the disease progresses these leaves develop dark brown to black spots. If the lower surface of infected leaves is examined, gardeners will notice that the leaves look dirty, or are covered with a thin layer of gray to brown fungal growth. This dirty looking fungal growth includes spores and spore producing structures of the downy mildew pathogen.

The downy mildew pathogen can move into a garden on infected seed, transplants or on wind currents. Unlike other downy mildew pathogens that prefer cool weather, basil downy mildew will tolerate cool weather but thrives in warm conditions. This means that the pathogen is actively growing and spreading for most of Minnesota's growing season.

At the end of April, University of Wisconsin plant pathologist Amanda Gevens reported identification of basil transplants infected with downy mildew, available for sale at a local store in Wisconsin. This early arrival of the pathogen in the Midwest could mean that more severe disease is found in the region this year as the pathogen will have time to grow and spread.

S. Jensen, Cornell University,

Photo 2: Dark growth on the lower leaf surface of a plant infected with basil downy mildew

Avoid downy mildew of basil by purchasing only healthy transplants and spacing plants to provide good air movement and quick drying of leaves after rain or dew. The basil varieties 'Spice' 'Blue Spice' and 'Blue Spice Fil' have been reported to have good resistance to downy mildew. 'Genovese', 'Martina', 'Italian Large Leaf' and 'Superbo' are all highly susceptible to the disease. 'Red Leaf' 'Red Rubin' 'Lemon' 'Lemon Mrs. Burns' 'Lime' and 'Sweet Dani Lemon Basil' were susceptible to downy mildew but had fewer symptoms than the most susceptible varieties.

If you suspect your basil plants are infected with downy mildew, please contact the Minnesota Department of Agriculture by sending an email with photos of the plant to or leave a voice message at 1- 888-545-6884. A MDA employee will respond in 1 to 2 days.

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