Green Algae Causes Leaf Spots on Avocados

  

Avocado trees, along with mangoes, can be attacked by a green algae called Cephaleuros virescens.  Most leaf spots are caused by fungi or bacteria. Occasionally, an alga will cause such spots with frequent rains and warm temperatures. The algae are dispersed by wind and splashing water. In addition to avocados and mangoes, about three hundred different plant species  are susceptible to this problem including guava, breadfruit, cacao, kava, tea (Camellia sinensis) and some citrus. 

Typically, green algae causes minor leaf spotting. Yet 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 or 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, the algae is called ‘red rust’.

Again, 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. Here are two recommendatons: keep the tree well fertilized and weeds under control. Copper fungicidal sprays, although usually not necessary, will control the disease.

Podocarpus Trees Attached by Beetles

The black twigborer (Xylosandrus compactus) is a beetle that attacks over 200 plant species including orchids, citrus, avocados, mango, coffee, eucalyptus, hibiscus and podocarpus. The females tunnel into woody twigs, leaving pin-sized entry holes. Once inside, they excavate galleries and lay eggs.  This excavation causes the damage to the tree. An infestation by one to three females is sufficient to kill the twig or branch. A severe infestation can kill a plant or even large trees. The damage is not caused by feeding, since the beetle larvae feed on a fungus that is introduced by the female beetle.

Typical symptoms of black twigborer are wilted and dead leaves beyond the beetle's entry hole. The dried, brown leaves frequently stay attached to the tree.

                                                    Entry hole

Cultural control
The best control is to maintain a vigorously growing tree. This will help the tree to resist beetle infestations and recover quickly from existing ones. Prune and destroy all beetle-infested plant material.

Corn Seedlings Hit the Ground

Don't think it's smooth sailing just because the corn seed has germinated and is up and growing.  Young seedlings are often found toppled over. There are three possible culprits for this dilemma.

The two most common offenders are birds and rats. Netting will help to exclude the birds and maybe a scarecrow, too.  Rats will come at night and nibble on the succulent seedlings. Bait or trapping is the solution for rats.

There are also some caterpillars like cutworms, that at night will feed on the seedlings, cutting off the stem at ground level. The plant is not consumed but damaged enough to cause the seedling to topple.

Removing old crop residue and surrounding weeds will reduce the cutworm population. Worms can be hand-picked from the ground at night. Spraying the seedlings and the ground with Bt (Bacillus thuringiensis), an organic pesticide,  may also help.

Photos: University of Minnesota Extension