Monday, April 27, 2015

Mango - Big Tree, Little Fruit

 Mango trees may grow well but produce little or no fruit. This condition is likely brought on by a disease called mango anthracnose (Colletotrichum gloeosporiodes).  During the bloom period, when the weather is warm and wet, mango blossoms are attacked by this fungus.  Blooms turn black and are destroyed and little or no fruit is set.  This disease also affects other fruit trees throughout the world including banana, avocado, papaya and coffee. 

The fungus also occurs on the leaves, twigs and fruit.  On the leaves, it is seen as small angular brown-black spots which can grow larger.  Sometimes the dead spots will dry and drop out leaving holes in the leaves.  Under wet conditions lesions on the stems may produce eye-catching pinkish-orange spore masses. Most of the infections on the green fruit remain latent until the fruit is ripe.  Then dark, sunken decay spots will develop and eventually will penetrate deep into the fruit. 

                                                        Anthracnose fruit symptoms

                                        Anthracnose fruit symptom know as tearstaining

What to do.  1.  Plant resistant varieties, especially in wet areas. The following varieties are listed as moderately resistant - Carrie, Earlygold, Edward, Glenn, Julie, Keitt, Florigon, Tommy Atkins and Van Dyke.   The one tree that shows the best resistance to this disease is Florigon.

2. Prune trees yearly and remove fallen plant debris from the ground. 

3. Spray with a copper fungicide when blooms first appear and continue until fruit is 1 ½ to 2 inches long.

It is difficult to grow mangoes in areas with high rainfall.  According to a UH publication, mangoes are best adapted to hot, dry leeward areas receiving less than 60 inches of rainfall annually.

Some mango varieties, especially Haden and Pirie, tend to alternate bear.  That is they produce a large crop one year, followed by an ‘off year’ with low yields.  

Photos: Dr. S. Nelson, University Hawaii

Monday, April 20, 2015

Air Layering

Questions often arise concerning air layering of plants. This is a method of reproducing a plant in order to create another exactly like the original. It has been used successfully as an alternative means of propagating some plants difficult to root, i.e. plants where grafting or cuttings have not been successful or where seeds do not come true to type.  Another advantage to air layering is that these plants have the ability to bear fruit much sooner than a seedling. Some fruit trees may take 10 years to fruit from seed. Plants such as citrus, figs, lychee, longan, mountain apples, camellias, azaleas, croton and dieffenbachia are frequently propagated by air layering and bear fruit (where applicable) in less time than seedlings.

How to Proceed
After selecting the branch you want, come down about a foot from the tip.  Here you will remove a one inch wide ring of bark by making two parallel cuts (1 inch apart) around the circumference of the shoot. This will leave the branch with a “naked” or barkless band.  Don’t cut through the branch, just cut deep enough so the outside bark peels off.  This is known as girdling.

Then wrap the wound with moist, unmilled sphagnum moss. You may need to tie some twine around it to keep it in place. Cover the moss with plastic wrap to form an airtight pouch. Alternatively, you may want to place the moss first, then wrap both moss and plastic around the wound at the same time. Make sure the moss is moist – not wet. Some gardeners will place a final wrapping of aluminum foil to shade the area from the hot sun. Also, some add a little rooting hormone to the wound to speed up the rooting process - a light dusting will do. For many trees, this is not necessary. 

Tie the bundle at each end, secure enough to prevent moisture loss.  Later, when roots are visible, all around the bundle, cut the branch just below the developing roots, remove plastic and plant the newly rooted branch in a pot.  Keep it in the shade until the plant develops more roots. Rooting time will vary depending on the type of plant and the season.  Some plants will begin to show roots in 4-6 weeks, others may take up to 2 years.

In the process of air layering, the removal of the bark (girdling) prevents carbohydrates from flowing down past this point, but still allows water and mineral nutrients to flow upward to the leaves. This keeps the leaves of the shoot from drying out, but the presence of the carbohydrates and the water in the sphagnum moss, will cause dormant buds in the area to grow into roots.

For optimum rooting, make air layers in the spring on shoots produced during the previous season, or in mid-summer on mature shoots from the current season's growth. On woody plants, stems of pencil size or larger are best. 

East Hawaii Residents:  Master Gardeners' Annual Plant Sale is Saturday, April 25, at 8:00am-Noon at the Komohana Agricultural Center. Nick will be there to answer questions.

Monday, April 13, 2015

The Whitefly Menace

Whiteflies are not only a menace to our gardens but to commercial agriculture as well.  There are more than 1,000 whitefly species in the world. Twenty species occur in Hawai`i; 12 of which have been accidentally introduced in the last 20 years.  They attack citrus, avocado, plumeria, poinsettia, anthurium and a host of vegetables and other ornamentals. When approaching heavily infested plants, clouds of adult whiteflies fill the air. 

The adult whiteflies, which are not true flies at all, look more like tiny, white moths.  They are sapsuckers, continually drawing off water and nutrients from plants, causing leaves to yellow, shrivel and drop prematurely.  Eventual death of the plants can occur.  Whiteflies also transmit several viruses. In addition, they cause an indirect decline of the plant by their heavy excretion of a liquid, rich in amino acids and sugars, called ‘honeydew.’  This liquid is constantly dripping to coat the leaf surface below, which is soon colonized by a common airborne fungus, called sooty mold. Although the fungus itself does not invade the leaf tissue, the leaf quickly becomes black and dirty looking, restricting respiration and photosynthesis in the plant.

Gardeners have great difficulty in controlling whiteflies because they are so prolific. Furthermore, different stages of growth have varying degrees of susceptibility to pesticides.  This means that after spraying, only a portion of the whitefly population will die.  Therefore, repeated spray applications are needed.  Also, some whitefly populations have developed resistance to pesticides.

The application of broad spectrum pesticides to control whitefly is generally not recommended, because this kind of spray has harmful effects on natural enemies (the good guys). Broad spectrum pesticides are those that affect a wide range of insects as opposed to those which affect the target species only.

So what do we do?  First of all, if tomato or bean plants are heavily infested, throw them away.  If a shrub is heavily infested, severely prune, and deal with the new population while it is still small.  Here are some other tips:
·        Yellow sticky cards – These should be placed adjacent or just above the infested plants – one card per 2 plants.  They have good potential for controlling light or new infestation. The insects are attracted to yellow and will stick to the card when they land.  Once the population has built up, however, the cards are not very effective.

·        Syringing - Apply a strong stream of water at the underside of infested leaves as often as possible.  This will wash off many of the whiteflies, as well as the honeydew.  University of California studies have shown this method performs as well or better than chemical treatments.

·        Pruning – Prune away severely infested portions of the plant.  Place material in sealed bags and dispose of properly.  Keep in mind that excessive pruning stimulates much new growth, which is attractive to whitefly.

·        Ants – Ants are a detriment to biological control and must be eliminated. The ants are bigger than many whitefly parasites and, by their aggressive behavior, will discourage the good guys from the scene.

·      PesticidesIf you spray, use insecticidal soaps and horticultural oils; this  minimizes the adverse effects of pesticides on the natural predators and parasites. 

·       Foils - Aluminum foil or reflective plastic mulches have been shown to repel whiteflies. 

Many parasites and predators of the various whiteflies have now been identified. Some of these have been imported from other parts of the world.  Here in Hawai`i, as well as in other states, parasites and predators are doing a good job in controlling this pest.  They will not eliminate the whitefly, nor are they an instant control.  And, for various reasons, whitefly can flare up and ruin a good crop. Controlling the ants and keeping dust to a minimum is important in encouraging the natural enemy population which will combat the whitefly menace.