Friday, December 30, 2011

Black Streak of Banana

 What causes the yellowing and streaking in banana leaves? 
 Banana plants are commonly infected by a fungal disease called Black Leaf Streak of Banana.  The leaves exhibit conspicuous yellowing, but upon closer examination you will find dark spots which enlarge to form the characteristic streaking effect. The streaks may vary in color. 

The principle means of spreading the disease is rain, which splashes the fungal spores to new leaves.  Fruit quality and yields can be reduced due to the disease.  It is unlikely that the banana tree will ever be completely free of the disease; however, pruning and disposing of infected leaves will keep the trees productive. Make sure the tree has adequate water and fertilizer to stimulate new growth; this will keep the tree one step ahead of the disease.  Also, removing some of the sucker growth will increase air flow in the canopy and reduce humidity, both of which increase severity of the disease.  Of the commonly planted commercial varieties, the dwarf Brazilian is less affected by black streak.

Monday, December 26, 2011

What does N-P-K mean?

What do the three numbers on a fertilizer package mean?   
All fertilizer packages have three numbers printed on them; for example, 10-15-5. These numbers represent the percentage of the three primary plant nutrients, nitrogen, phosphorus and potassium.  In the above example, the fertilizer would contain 10% nitrogen, 15 % phosphorus and 5% potassium.  This is important to know because different plants and different situations require different types of fertilizer. 
·        Nitrogen is important in the photosynthesis process and for many growth and developmental processes.  It moves readily in the soil and roots have easy access to it.  But during periods of heavy rainfall, the nitrogen is leach beyond the root zone and can contribute to groundwater pollution.  Deficiency symptoms include stunted growth and chlorotic (yellow) and pale green leaves.  These symptoms usually begin in older tissue because nitrogen is mobile within plants and moves from older to younger tissue when in short supply.  Plants with excess nitrogen will exhibit dark green leaves, reduced yields and delayed maturity.
·        Phosphorus stimulates early growth and root development and promotes the formation of seeds.  It is also important for the energy systems of the plant.  Soil phosphorus is often tied up chemically in relatively insoluble compounds and therefore does not leach as much as nitrogen will.  Phosphorus fertilizer uptake by the plant is more efficient when in the presence of nitrogen.  Although some Hawaiian soils have adequate levels of phosphorus, many are low in this element.  Deficiency symptoms include slow growth, dark green leaves and a purplish color on older leaves.  Excess phosphorus can interfere with the uptake of micronutrients.
·        Potassium is involved with cell division, starch and protein synthesis and sugar movement within the plant.  It increases size and quality of fruits and vegetables and increases disease resistance.  Potassium is usually abundant in soils, but much of it is tied up in soil minerals and is unavailable to plants.  Soils in high rainfall areas are usually low in potassium. Deficiency symptoms include slow growth, small fruits and chlorosis on the leaf tip and margins.

You can buy fertilizers containing a single nutrient, like ammonium nitrate and urea, containing 33% and 45% nitrogen, respectively. Or, one containing a double nutrient like ammonium phosphate, which is 11% nitrogen and 48% phosphorus.  You can also purchase what is called a complete fertilizer which contains all three primary nutrients.  The term complete should not be interpreted to mean that the fertilizer supplies all the nutritional needs of the plant.

Many plants have similar nutritional requirement, but many have differing requirements.  A lawn, which is grown for its foliage and not fruit or flowers, will basically need nitrogen applications, with P and K added only occasionally, depending on their soil content.  Bananas require ample potassium; a fertilizer similar to 10-5-20 would be appropriate.  Papayas do well with a more balanced fertilizer. Vegetables in general like a high phosphorus content, like 10-30-10.  In determining which fertilizer to use, it is always best to have the soil analyzed; at least the primary nutrients, N-P-K.

Friday, December 23, 2011

Grapefruit Melanose

What causes the brown speckling on the skin of grapefruit?  The staining is a fungal disease known as melanose. This disease displays a very interesting pattern on the fruit called ‘tear staining.’ It is caused when dew, contaminated with spores, runs down over the surface of the fruit. As this happens, the fungal spores can stick to the skin of the fruit. These spores germinate and cause tiny spots on the fruit in the pattern of the water running down the fruit, hence the name ‘tear staining’. 

Unless the disease becomes very severe, melanose is considered a cosmetic problem and, in commercial operations, will cause a downgrading of the fresh fruit. 
For the homeowner, the fruit is just as good inside as any perfect looking piece of fruit at the supermarket.

With an abundance of rain, the spores splash to nearby leaves and twigs, causing leaf drop and twig dieback.  Many of the dead twigs we see in citrus trees are caused by this fungus.  (Note: In high numbers, scale insects can also cause twig dieback.)

Although a tedious job, it is best to prune out all the dead twigs, they are store houses of the fungal spores.  However, whether you prune or not, the trees will not die from melanose.

Monday, December 19, 2011

Papaya Fruit Shape

 Why do papayas  produce different shaped fruits?  Are these different varieties?  The different shapes are basically due to the fact that there are three different types of trees, not necessarily due to different varietal types.  There are female plants, which produce only female flowers - these contain an ovary for fruit production. There are also male plants which produce male flowers - containing only pollen.  Then there is the ‘hermaphroditic’ plants which contain only hermaphroditic flowers.  That is, flowers that contain both ovary and pollen and can pollinate each other, called self-pollinating. 

The female trees must be pollinated in order for fruit to mature.   This fruit is a medium to large, round-shaped fruit of good eating quality and a large seed cavity. Unpollinated female flowers develop small fruit, which drop to the ground.  Male flowers rarely form fruit.  Hermaphroditic plants consistently produce small to medium, elongated to pear- shaped fruit also of good quality with a smaller seed cavity.

Monday, December 12, 2011

Using Soap to Kill Insects

 Is soap good to kill insects in the garden?  
 Yes it is, in fact, gardeners have been using soaps as an insecticide since the early 1800’s.  But there are some qualifications.  An insect wears its skeleton on the outside – called an exoskeleton.  This is the hard shell we see on many insects like beetles, grasshoppers and cockroaches.  Soap will breakdown this exoskeleton, causing desiccation and thus killing the insect.  But soap has no residual affect and therefore must be sprayed directly on the pest.  Soaps will not kill insects that land on plants after they have been sprayed.
The other problem with soaps is that some plants are very sensitive to them and could exhibit some burning on the leaves - called phytotoxicity.   This is especially true if you are mixing your own concoction.  Use only a mild dishwashing soap with no additives. Purchased products are well refined and would have fewer problems with phytotoxicity.    
Here is a University of Hawai`i recipe for a good oil and soap mixture:  Make a concentrate of one tablespoon dishwashing liquid and one cup of vegetable oil.   When ready to use, shake well and mix 1 – 2 ½  teaspoons of the this mixture into one cup of water.  Spray plants thoroughly, every 5 – 7 days as needed.  Don’t spray in the heat of the day.

Friday, December 9, 2011

Cashew trees

Cashew trees are attractive, fast growing trees with pink flowers. They are known for the nuts they produce. But this nut is produced outside of the fruit, which is known as the cashew apple. The nut is attached to the lower portion of the cashew apple. Botanically speaking the apple is actually the swollen stem of the fruit.

Cashew apples and cashew nuts are excellent sources of nutrition. The cashew apple contains five times more vitamin C than an orange and more calcium, iron and vitamin B1 than citrus, avocados and bananas.

The best soils for growing cashews are sandy soils, but the trees grow well even in marginal ground. You can grow cashew trees from seed, but they will not be true to type – they will not be identical to the parent. Seedling trees flower in the third year after planting. Grafted or air layer cashew trees can fruit within 18 months, and the fruit (and other tree characteristics) will be identical to the ‘mother’ tree.

Cashew apples and nuts are ripe about two months after fruiting. The apple will be pink, red or yellow, and the shell of the nut will turn a dull grey. When the fruit falls to the ground, it's definitely ready. The nut develops first while the apple develops and enlarges only 2 weeks before the fruit falls. In Brazil, fresh cashew apples are packed in trays and marketed in retail fresh produce outlets.

As with mangos, rain during the flowering season will cause flowers to drop due to the anthracnose fungus. There are more than 200 registered patents for different uses of the oil contained in the shell of the nut. One of the most important uses is in the manufacture of brake linings.  There is also a caustic liquid that causes severe burns inside the shell of the nut. Be careful when handling it.

Wednesday, November 30, 2011

Sweetness in Citrus

  Will citrus fruit get sweeter by adding potassium or lime to the soil?
Although the notion of adding potassium and/or lime to sweeten citrus fruit has been around for a long time, it has not been substantiated by scientific research.  Fruit from trees deficient in potassium are small and thin-skinned but are juicy and have a good acid and sugar content.  Adding potassium to deficient trees will increase the size of the fruit.  This has also been noted in other fruit trees, such as apples and peaches.  An excess of potassium makes fruit large with coarse rinds, thick skins and poor eating quality.
 A classic chart taken from the University of California’s Citrus Industry Volume II, shows that as the percent of potassium in the leaf rises, there is a slight decrease in the sugar content of the fruit’s juice.  However, the vitamin C content increases.
If soils are truly potassium deficient, by all means apply potassium.  However, sweeter fruit will not be a result of adding potassium.

In regards to lime (calcium carbonate), an old study (1958) in South Africa, on calcium deficient orange trees, noted that the fruit was acidic and low in sugars. The trees were stunted, chlorotic (yellow), sparsely foliated and exhibited profuse blooming and excessive young fruit drop.  If the soil is truly calcium deficient, adding lime may increase the sugar content of the fruit. A soil analysis can determine whether the soil is deficient in calcium, as well as other nutrients.

Monday, November 28, 2011

Citrus pruning

Lemon trees grow vigorously, especially the Lisbon variety. If not pruned on a regular basis they will develop a very dense canopy, allowing little light to penetrate into the interior of the tree. Under this condition, the tree will produce few interior blooms and subsequently a small amount of fruit.  In addition, pesticide sprays become ineffective because of the inability of the spray to penetrate the dense foliage.  Left unpruned, a lemon trees will also become quite tall, making picking difficult. 
Other citrus trees like orange and grapefruit are not as vigorous in their growth, and therefore do not need as much pruning as lemons. These trees can be left unpruned without serious consequences. If desired, a large tree can be pruned by simply removing a few of the interior branches to open the tree up to more light. Note: studies have shown that yields are reduced in proportion to the severity of pruning. To rejuvenate an older tree, pruning may range from moderate thinning of the canopy to heavy pruning. This pruning will stimulate a new flush of growth; however, the recovery will be temporary unless the cause of the trees decline has been corrected.
When to prune
 The timing for pruning is not critical.  After the last piece of fruit is removed for the season, is generally a good time.  Citrus like Valencia oranges and some mandarins tend to alternate bear. That is, they produce heavily one year followed by light production the next.  In this case, it would be best to prune during the ‘off’ or light year. In addition, if you have the time, it is prudent to remove the dead twigs from a tree. These harbor disease spores which, during a rain, will splash and infect nearby fruit causing the fruit to blemish. 

Friday, November 25, 2011


Inconsistent fruit production in lychee.
 When it comes to producing fruit, lychee trees, as well as longans, are very temperamental.  In order for the tree to bloom profusely, there are certain requirements that need to be met:  a combination of a dry period, usually about 4-6 weeks, along with low temperatures. The cool temperatures then need to be followed by warm weather for good fruit development. Among the many varieties of lychee, kaimana requires less cool temperatures and is therefore a good variety for warm areas.
 It is apparent that in order for lychee trees to produce well, they need a very specific climate. When this is not met, inconsistent fruit production will occur.

Monday, November 21, 2011

ALBIZIA – Beautiful tree or a true menace?

The albizia trees that quickly spring up in vacant lots or recently cleared land have become one of Hawaii’s worst invasive species. It has been called  “the tree that ate Puna”.

This albizia tree, Falcataria moluccana (it is also named Albizia falcate, A.moluccana and others), is a native to such places as Papau New Guinea, the Bismark Archipelago, and the Solomon Islands. The wood is used for light weight construction, cabinets, furniture, toys, and match sticks.
It is a fast growing tree to over 100 ft. tall. The tree can reach heights of more than 20 feet in the first year and to 60 feet by the end of ten years. It is described as a deciduous tree with wide-spreading branches, capable of shading over half an acre.  The tree produces large seed pods 4-5 inches long and about ¾ inch wide. It grows from sea level to about 3,200 feet elevation.  It is a nitrogen fixer, meaning that with the help of some bacteria residing in its roots, it can pull nitrogen out of the air and convert it to a form that roots can absorb. Because of this ability, it is used in many countries to improve soils as well as provide shade in coffee plantations.

This may make it sound like a pretty decent tree. But Dr. J.B. Friday, Extension Forester with the University of Hawaii, states “In Hawaii this tree is invasive in native`ohi`a forests as well as on land disturbed by human activities (especially bulldozing). It will causes the`ohi`a  to die off and makes it easier for other invasive species like strawberry guava, clidemia and possibly miconia to grow under it and further degrade the forest. It also improves habitat for the coqui. It is a distinct threat to our native wet lowland forest ecosystems such as there are in Puna.”

Because of their brittle wood and weak structure, these trees are an even more serious problem to homeowners. Strong winds can cause large limbs to drop onto whatever may be underneath them, causing costly damage to homes, other structures and power lines. The fragile branches often cause traffic hazards by dropping branches on the highway.

A major problem, especially in Puna, is that people sometimes hire a bulldozer to clear their land. They clear the native ohia/uluhe forest, which is somewhat resistant to albizia invasion. If they are absentee landowners, they may not return for a few years and when they donreturn, an albizia forest has sprung up. Albizia is much bigger and faster growing than ohi`a, so the native trees cannot compete. To complicate matters, there is now a source of seedpods for the alibizia to invade the neighbors' land. Dr. Friday advises not to clear the native forest on your lot unless you have another use planned for it. Otherwise albizia will come in and take over and you'll have a huge problem

What to do: Leave the natural vegetation untouched—don’t bulldoze—until you are ready to utilize the land. Eliminate albizia seedlings and small trees before they become a problem.
Methods of control:  
After trees are cut down, immediately apply herbicide to cut stumps. Triclopyr amine is effective at concentrations of 7-10%.  For larger trees near buildings it may be advisable to first contact an arborist to determine the safest and most effective course of action to take in removing the tree.

The US Forestry Service in Hilo has an excellent publication entitled “Albizia – The Tree That Ate Puna” authored by C. Sumida, F. Hughes and Kathleen Friday.   For a copy contact Veronica at or write to USDA Forest Service, Institute of Pacific Islands Foresty at 60 Nowelo St, Hilo 96720; or call at 933-8121       

 Ask the Garden Guy, Science Based Answers to Garden Questions, is an excellent resource book for gardeners. Some popular topics include Slugs and Snails, Organic Pesticides, Reasons, Why Vegetable Seeds Do Not Germinate, What’s So Hot about Manure? Mushrooms in the Lawn.  Purchase by clicking on the image of the book above.