Monday, July 27, 2015

Avocado Flower Types

Avocado varieties are categorized as A and B flower types.  All avocado flowers have both male and female components (stamens and pistil). But an unusual and unique facet of avocado flowering is that they open as a male flower at one time of the day and open again as female at another time. Here is the scenario: ‘A’ type flowers open female in the morning, close, and then reopen as male in the afternoon. ‘B‘ type flowers are male in the morning and female in the afternoon. This works out very well in a garden with both A and B type trees. In the morning, the female A type flower will be pollinated by the B type males. And in the afternoon, just the opposite will occur.

If there were only one type of avocado tree present in the garden, potentially little fruit would be produced. Yet the presence of bees, with their efficient distribution of pollen, will enable the single backyard tree to produce an abundance of fruit. In addition, there is usually, although not always, the opposite type tree somewhere close in the neighborhood. The bottom line for backyard growers is that in most cases one tree will provide ample fruit.

Commercially, much research has been conducted on mixed orchards, i.e., fields containing both A and B flower type avocado trees. Fruit production increased in the mixed orchards. 

University of Hawaii CTAHR has an excellent publication available online entitled “What Makes a Good Avocado Cultivar Good?” Table 2, in this publication shows various avocado cultivars grown in Hawaii and their flower type. For example, the Sharwil variety, which has a bearing season of winter - spring, is a B flower type;  Greengold, with a bearing season of winter – spring and Ohata, which bears in the summer are both A flower types. The CTAHR (College of Tropical Agriculture and Human Resources) website can be found at
Other websites such as the University of California also have excellent information on avocado flowering types,

Saturday, July 18, 2015

Organic Weed Control

Organic herbicides (OH) include products with ingredients such as acetic acid (vinegar), citric acid, d-lemonene, in addition to clove, cinnamon and lemongrass oils. These products are classified as contact herbicides, meaning they will damage any green vegetation they contact; yet they have no residual activity. OH are not absorbed into the plant system and are safe when sprayed on woody stems and trunks. The foliage of perennial weeds will burn when sprayed, but the plant will recover; repeat applications are needed.

The organic herbicides are effective in controlling weeds when they are small; weeds in the first true leaf stage are easier to control than older weeds. Environmental conditions such as temperature and sunlight may also influence the effectiveness of these herbicides. Many of the products work better when temperatures are above 75F. Furthermore, broadleaf weeds are easier to control than grasses. 

No matter what the situation, 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 the spray mixture to enhance the performance of the pesticide. As a final note, organic herbicides are expensive.

Monday, June 29, 2015

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.