Saturday | January 21, 2017
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Many varieties of plants susceptible to green algae

Garden Guy, I have waited seven years for avocados to appear on my trees. Now there is small fruit galore. I have fertilized and mulched as you have suggested and the leaves are dark green with some new growth. But some of the lower leaves have reddish spots … the same thing is showing up on my one mango tree. Will the discolored leaves affect the fruit? Do you think my trees are diseased? Help needed in Hilo.

Don’t panic yet. It sounds like the trees have a minor attack of green algae, Cephaleuros virescens. Most leaf spots are caused by fungi or bacteria. But occasionally an alga will cause spots on leaves, prompted by frequent rains and warm temperatures. The algae are dispersed by wind and splashing water.

Close to 300 different plant species are susceptible to this problem including mangoes, avocados, guava, breadfruit, cacao, kava, tea (Camellia sinensis) and some citrus.

Typically, green algae is of minor importance as it causes only some leaf spotting. Poor plant nutrition, poor soil drainage and stagnant air can predispose a tree to infection.

On avocado and mango, the leaf spots appear burnt-orange to rust colored. On guava, the leaf spots are more brown/ black, often with a yellow halo around each spot.

The fruit can also be spotted, causing a scabby or scarring appearance, often affecting the marketability of the fruit. The algae do not cause a soft rot of the fruit.

When attacking coffee, it is called “red rust.” With avocados and mangoes, there is generally no damage to the plant’s vigor or yield. Treatment is rarely needed. Leaves on low hanging branches are mostly attacked.

If desired, the low-hanging branches that are affected may be pruned away. Keep the tree well fertilized and the weeds under control. Copper fungicidal sprays, although usually not necessary, will control the disease.

I was able to buy a few grape trees today, seedless Red &Thompson Green. I live in Panaewa and have good drainage and lots of sun on this side, too. Hope to become a vintner. — Donna

I don’t think you’ll be competing with Napa Valley, but Thompson seedless variety should grow and give some fruit. They, along with many other grape varieties, have a low chilling requirement, around 100 hours. Without cool temperatures, the overall yield of the vines will not be plentiful. See my website, and search “deciduous fruit trees.” This will give you an explanation of chilling requirement.

Also check out the University of California website and for information on all aspects of growing grapes.


Weeds Organically

Organic herbicides include products with ingredients such as acetic acid (vinegar), citric acid, d-limonene, clove, and cinnamon and lemongrass oils. These products are classified as contact herbicides meaning they will damage any green vegetation they contact; they are not absorbed into the plant system. These herbicides are safe when sprayed against woody stems and trunks; they have no residual activity. Perennial weeds may be burned back, but they will recover; repeat applications are needed.

Organic Herbicides are effective in controlling weeds when they are small and environmental conditions are optimum. It has been suggested that temperature and sunlight are factors affecting the effectiveness of these herbicides. Many of the products work better when temperatures are above 75 degrees. Weeds in the first true leaf stage are easier to control than older weeds. Broadleaf weeds are also easier to control than grasses. For both types, good coverage is essential. Adding an organically acceptable spray adjuvant to the herbicide mix has resulted in improved control. An adjuvant is a nonpesticide material added to spray mixture to enhance the performance of the pesticide. As a final note, organic herbicides are expensive.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at Also, visit his website at


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