Latin name: Septoria rubi
Reason for Concern:
1) Common severe on erect and trailing blackberries and black raspberries.
2) Leaves and canes of severely infected plants become badly spotted.
3) Cause premature defoliation, which produces weak plants susceptible to winter injury.
On leaves, Septoria Leaf Spot lesions have a whitish to gray center surrounded by a brown to purple border (at first, they are purplish then later turn brown). The spots are circular, about 3 or 4 mm in diameter. Tiny black fungal fruiting bodies form in the center of the spots (use a hand lens to see them). Leaf spots are similar to those of Anthracnose, but generally are more circular, while anthracnose lesions are irregular. Leaf spots may also resemble Blackberry Rust, but have no yellow pustules on the lower leaf surface. Spots on canes and petioles are similar to those on leaves but are more elongated.
The fungus overwinters in dead plant debris and on infected canes. Splashing or wind-driven rain releases spores in high numbers and carries them to young susceptible leaves and canes. The fungus germinates in a film of moisture and penetrates leaf or cane tissue. As leaf and cane spots form and age, new fungi form in the centers. These also produce and release spores that can cause secondary infections throughout the growing season. Although environmental conditions required for infection are not clearly understood, periods of rainfall are highly conducive to disease development.