Problem type: Disease

Name of problem: Leaf spots

Plant name(s): All deciduous plants

Symptoms / Characteristics:
Leaf spots are caused by a number of fungi or bacteria that produce lesions from tiny spots to large blotches on leaf surfaces. Spots can be circular to irregular in shape and range in color from tan to red, purple, brown or black. Some spots may be two-toned or have a different colored ring around the margins or in the center of the spot. Dead tissue in the center of lesions may fall out creating a shot hole appearance or a ragged leaf. Some leaf spot diseases produce blister-like lesions or tar spots that are quite conspicuous on leaf tissue.

Spots may remain small and the plant can localize the infection so that even if lesions are numerous, little harm is done to the plant. Other pathogens might expand the lesion into large blighted areas and in severe cases, cause defoliation or premature leaf drop. Spread of leaf spot diseases is primarily through water rather than air and will persist if conditions are favorable. Black spot on roses is an extremely common leaf spot, which occurs wherever roses are grown. Generally leaf spot diseases occur sporadically and cause little damage.


Control / Preventions:
Collect fallen leaves before winter to reduce overwintering sites of the pathogen, which can invade new growth in the spring. Where the number of infected leaves are small, individual leaves can be removed or branches pruned where infections are localized. Keep leaves as dry as possible by thinning the crown if too dense, watering in the early morning and adjusting sprinkler spray height to avoid wetting the leaves. Space plants appropriately to reduce humidity and increase air movement.

Chemical control is usually not warranted as it will not cure the leaves once already infected. Fungicides applied before the disease appears and on a regular basis throughout the season can provide effective control.


Photos:
leaf_spot_on_dogwood1


Relevant web sites:
www.forestpests.org/cgi-bin/searchthumb.cfm?searchparam=leaf%20spots


Other references:
Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987. Diseases of Trees and Shrubs. Comstock Publishing Associates, Cornell University, Ithaca, NY. 574 pages.

Hiratsuka, Y. 1987. Forest Tree Diseases of the Prairie Provinces. Canadian Forest Service. University of British Columbia Press. 142 pages.