What causes the yellowing and streaking in banana leaves?
Banana plants are commonly infected by a fungal disease called Black Leaf Streak of Banana. The leaves exhibit conspicuous yellowing, but upon closer examination you will find dark spots which enlarge to form the characteristic streaking effect. The streaks may vary in color.
The principle means of spreading the disease is rain, which splashes the fungal spores to new leaves. Fruit quality and yields can be reduced due to the disease. It is unlikely that the banana tree will ever be completely free of the disease; however, pruning and disposing of infected leaves will keep the trees productive. Make sure the tree has adequate water and fertilizer to stimulate new growth; this will keep the tree one step ahead of the disease. Also, removing some of the sucker growth will increase air flow in the canopy and reduce humidity, both of which increase severity of the disease. Of the commonly planted commercial varieties, the dwarf Brazilian is less affected by black streak.