Some causes of leaf spots
Air pollution
Aggravates other causes if air quality is poor.
Anthracnose
Many hosts; promoted by moisture during new growth.
Bacterial blast, blight, and canker
Dieback, cankers, and oozing twigs may be associated symptoms.
Chewing, mining, or sucking insects
Insect presence usually helps to identify.
Entomosporium leaf spot
Plants in Pomoideae group of the Rosaceae are affected.
Eriophyid mites
Tiny, elongate mites may barely be seen with a hand lens.
Mineral deficiency or toxicity
Often produces characteristic pattern of discoloration helpful in identifying the cause.
Pesticide injury
Commonly herbicides, but other pesticides can be the cause.
Rusts
Orangish or yellowish spore masses, usually on leaf underside or on branches or stems.
Scabs
Dark, circular, scabby or velvety fungal spots on many hosts.
Scale insects
Unlike diseased tissue, scales can usually be scraped off.
Septoria leaf spot
Spots mostly older leaves; cankers may develop on poplars.
Shot hole
Almond, apricot, plum, and other Prunus spp. are affected.
Spider mites and red mites
May be webbing, foliage speckling, or tiny mites present.
Sucking insects
May be dark excrement of thrips, lace bugs, or plant bugs.
Sunburn
Yellow or brown area beginning between leaf veins, mostly on southwest side of drought-stressed plants.
Taphrina leaf curl
California buckeye and oak leaves affected by Taphrina fungi if spring is moist.
Viruses
Streaked, discolored, or distorted foliage.
Water deficiency
Often preceded by leaf wilting.
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