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Symptoms of BLSD and SD are sometimes difficult to differentiate.
In general, the first symptom is the appearance on the upper leaf surface of pale yellow streaks (SD) or dark brown streaks on the lower leaf surface (BLSD), both 1-2 mm long which enlarge to form necrotic lesions with yellow haloes and light grey centres. Lesions can coalesce
and destroy large areas of leaf tissue which results in reduced yields and premature ripening of fruit.
    BLSD is more serious than SD because symptoms appear on younger leaves, wh i ch is generally due to a greater amount of inoculum, and hence more damage is caused to photosynthetic tissue. BLSD also affects many cultivars that have resistance to SD, such as those in the plantain subgroup (AAB). Yield losses of up to 50 % have been reported in some cases.