With summer coming, be on the lookout for plant disease

Updated May 31, 2022

It is almost summer and for all the good things the season brings, it’s also a time when landscapers need to be on the lookout for certain diseases that typically rear their ugly heads during the summer months.

Weather in your region will play a role in determining whether some diseases become particularly problematic this year.

Powdery Mildew

How do I identify it?

Photo: Scot Nelson/FlickrPhoto: Scot Nelson/Flickr

This fungal disease is easy to spot, as it covers a plant’s leaves in a coating of white fuzz. There are many different species of this disease and it can infect numerous plants, including azaleas, lilacs, roses, dogwoods and even turfgrass.

Don’t confuse powdery mildew with downy mildew. Downy mildew damages plants far more than powdery mildew. It creates yellow or dark spots on the upper surface of the leaf and has fluffy gray masses that grow on the underside.

What causes it?

Spores move from plant to plant via the wind and powdery mildew favors high humidity and temperatures between 60 and 80 degrees Fahrenheit. This disease thrives in the shade and among crowded plantings with poor air circulation.

How can I solve it?

Provide sunlight and good air circulation to plants by ensuring proper plant spacing and pruning. Cut off and remove diseased plant debris to prevent further spread. Fungicides can be used to control the disease, but there aren’t any that can completely eradicate it.

When adding new plants to a landscape, select cultivars with a resistance to powdery mildew. Water can be used to inhibit spore germination for most powdery mildews, but only use this method if other leaf diseases are not a problem.

Brown Patch

How do I identify it?

Photo: L.P TredwayPhoto: L.P Tredway

Brown patch can be found from spring to fall, but summer is when cool-season lawns are at the most risk. This fungus creates irregular circular patches that can be 1 to 3 feet in diameter. The infection will at first appear water-soaked but will turn brown later on. In the morning, the outer ring may appear grey, known as a “smoke ring.”

This disease can be confused with Pythium blight, which creates Âľ to 2 inch in diameter brown spots that grow rapidly. These spots can form white masses of mycelium and irregular blots of dead turf.

What causes it?

Brown patch is caused by the fungus Rhizoctonia solani and is most active during warm, wet conditions when nighttime temperatures are above 68 degrees Fahrenheit and leaf wetness lasts longer than 10 hours.

Outbreaks can be more severe when nitrogen fertilizer has been used excessively during disease-favorable weather.

How can I solve it?

Scheduled irrigation from midnight to early morning will minimize long dew periods and the use of nitrogen fertilizer should be carefully monitored. Avoid using highly susceptible turf species, such as tall fescue in humid areas. Mow at a higher height, as low mowing heights can further develop brown patch.

Summer patch

How do I identify it?

Photo: North Carolina State UniversityPhoto: North Carolina State University

Symptoms of summer patch will appear initially as small circular or oval patches that are 1 to 3 inches in diameter. These patches will be straw-colored or reddish-brown. The size of the patches can grow to 1 to 3 feet wide. Less susceptible turfgrass species have been known to grow in the diseased patches.

What causes it?

Magnaporthe poae is a root, crown and stem disease that targets Kentucky bluegrass, annual bluegrass, and fine fescue. It is commonly found in lawns with compromised root systems, such as when sod or thick layers of thatch are left on poorly prepared soils. Locations with high soil moisture, compaction, poor drainage and high soil pH are prime candidates for summer patch.

How can I solve it?

It is important to relieve compaction with core aerification in the spring and fall. Apply acidifying fertilizers and increase mowing height.  Improve the drainage of the turf and use prolonged, infrequent irrigation.

Aster yellows 

How do I identify it?

Photo: Missouri Botanical GardenPhoto: Missouri Botanical Garden

A major symptom of aster yellows is chlorosis, where the leaves yellow while the veins remain green. Growth slows and flowers can appear deformed and can have leaves growing inside the flower. Seeds and fruit do not develop. Since the phytoplasma that causes aster yellows affects over 300 species, the symptoms can vary greatly. It can be mistaken for herbicide damage.

What causes it?

Phytoplasma is a bacteria-like organism that is spread by aster leafhoppers, which suck on the sap of plants and spread the disease with their saliva. It is most prolific during cool, wet summers. Hot, dry weather discourages the phytoplasma and leafhoppers.

How can I solve it?

Infected plants should be discarded immediately to prevent further spread. Since controlling leafhoppers is near impossible, it is wiser to grow less susceptible plants such as geraniums and impatiens. Weeds such as dandelions and plantains should be removed as they are known to harbor the disease.

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