Photo by Karen Rane, Plant Disease Diagnostician

Leaf Spot

 

Major Problems in Elmore County

By

Mir-M. Seyedbagheri

 

Summer is here!  Lawns and gardens abound with colorful flowers and tender young vegetables—and a few undesirable intruders as well.  The Extension office is bombarded daily with widely diverse pest and diseased plant samples for identification and control measures.  Many of the calls that I have received so far this year have been for two problems in particular: Leaf Spot (a plant disease caused by fungus, virus or bacteria) and the borer (a wood-boring pest that attacks weakened trees).

Leaf spot is caused by a fungus that infects new leaf growth in the spring. The leaf spot fungus manifests as small dark red or purplish spots that form on infected leaves.  The spots enlarge to about 1/8 inch in diameter with whitish centers and reddish margins, and the leaves gradually turn grayish to almost white.  Scattered widely over the leaf surfaces, these spots interfere with leaf functions. 

 

Fungus infections occur during moist weather, and are most severe in spring and fall. The disease is greatly aggravated by wet conditions, and certain cultural practices will help reduce infections:  Spacing plants so as to enhance rapid drying of the foliage after rain and watering; avoiding sprinkler irrigation, if possible, or watering early in the day; picking fruit at least every other day and removing rotted fruit; and spreading clean straw under your plants can all help reduce leaf spot infection. Leaf spot fungus survives the winter on old, infected leaves, and will reinfect the diseased plant the following spring unless the old leaves are raked out and removed in the fall and early spring before new growth begins.  Rake out and remove dead leaves and debris in early spring before new growth starts.  Fungicides are registered to control leaf spot if the problem becomes severe.  You can check with your local garden store for a fungicide that best works for your particular problem, and remember to read and follow label directions very carefully.

 

Some trees, shrubs and plants are more susceptible than others to this disease.  For example, the aspen tree is particularly susceptible to black leaf spot fungus which infects young leaves in the spring just as they are beginning to grow.  Black spots appear on the leaves and enlarge and spread, and can also cause the leaves to turn yellow and fall off.  Again, it is important to clean and remove all fallen leaves at the end of the year, and burn them if possible, so that they don’t serve as a source of re-infection.  However, if your neighbor has infected aspens and fails to clean and remove his leaves, spores from the infected leaves can re-infect your trees.  Spraying with Daconil, Propiconazole, or other fungicides in the early spring just as aspens begin to leaf out will effectively control black leaf spot.  It is important to spray thoroughly so all leaves are covered; because once leaves are infected, later spraying is not effective.  Again, carefully read and follow label directions.

 

Overall, the best offense is a good defense against any type of intruder in your garden, be it insect or disease.  Know your garden--proper nutrition and watering, and a vigilant eye for the first sign of any stress, disease or insect problem will go far to ensure healthy plants.  If you encounter any sign of pest or plant disease damage, address it immediately.  The public is encouraged to call and bring in samples to the University of Idaho Elmore Extension Office 208-587-2136 Ext. 509, for plant disease and pest identification and control measures.

 

*Please note that any brand name mentioned in this article is given as an example only, not a recommendation. For more information you are welcome to call the Extension Office 587-2136 Ext. 509. Or send an email: elmore@uidaho.edu.