The Leaves of My Hass Avocado Tree Are Turning Black

Cercospora Leaf Spot

• Caused by the fungus Cercospora purpurea, Cercospora spot produces tiny spots with yellow "halos" on leaves. The spots, less than 1/10 inch wide, are usually angular rather than round and so dark they may appear black. As the infection worsens, the spots darken further and merge to form larger blotches. This fungus can also infect the avocado fruit. Spores spread on the wind or through water. Insects can also carry the infection from plant to plant. Fungicides may work best as preventatives. Look for fungicides containing copper oxychloride and begin spraying in late spring. Follow label directions for application intervals and amounts.

Scab

• If the infected foliage is near the top of the avocado tree's canopy, the problem may be scab. Caused by Sphaceloma perseae, the fungus is more visible when it infects the fruit, producing raised, oval lesions that coalesce and become sunken as the disease becomes more widespread. On the undersides of leaves, these spots often take on a star shape and the centers fall out, leaving what is called shotholes. Scab thrives in cool, moist environments and most often attacks new growth. The foliage becomes resistant after about a month, but the fruit may still be affected without chemical treatment. Use a copper-based foliar spray to control scab on trees in the garden or in containers.

Phytophthora cinnamomi

• An oocyte, Phytophthora cinnamomi causes fungal-like infections of the root and crown of avocados. The pathogen can persist in the soil for long periods and interferes with the tree's ability to circulate nutrients and moisture from the roots to the foliage. The foliage wilts and turns brown to black before falling from the tree. Control is difficult, especially in container-grown Hass avocado trees. Use a fungicide containing Fosetyl Al, metalaxyl or etridiazol on the tree, disinfect the container and use fresh, sterile growing medium to repot the tree. Outdoors, plant avocados on raised beds in sandy soil. Lower the soil pH level to around 3.3 to inhibit zoospore production.

Prevention

• Practice strict sanitation to eliminate hiding places for fungi and other infections. Remove all plant debris and weeds and dispose of the material away from the garden. Disinfect pruning tools in between cuts to avoid spreading infection. Water the avocados from the bottom; moisture on foliage creates a breeding ground for infection. Do not overwater. Too much water can cause root rot, which in turn may open the door for infection and disease that affects the upper parts of the tree. Cut back on nitrogen levels in fertilizer; high nitrogen levels may leave the avocado tree susceptible to phythophthora cinnamomi.