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.

Friday, December 16, 2011

Crab Spiders

 Each fall local gardens are invaded by crab spiders, also known as spiny-backed spiders.  There are two species of crab spiders on the Big Island of Hawaii, both are shaped like tiny crabs approximately ½ inch across. One is white with black spots and 6 red spines. The more common one also has spines, although shorter, with two distinct white spots on its speckled yellow-brown back.  At least in Hilo, Hawaii, crab spider populations increase in the fall and decline in the late spring.

These spiders are different from other spiders in their web building techniques. Their webs are interconnected, building condominiums right outside your door, under the eaves, between the shrubs and in the trees.  They seem to be spun everywhere, but the spiders are not a nuisance indoors. 

People, especially farm worker, have been bitten when the spider falls on them or gets trapped within their clothing.  Females lay eggs away from their webs, on the leaves of trees and elsewhere. The egg masses look like a flat patch of fluorescent green cotton candy which turns yellow when the eggs hatch.  Disposing of this egg mass will, of course, help to reduce the spider population.

Spiders, including the crab spiders, are some of the best bio-control agents known, because they feed on large quantities of insects such as aphids, ants, thrips and termites. They should be tolerated as much as possible. But, people do have their limits.  A soap and/or oil solution will kill them. In addition, many  household, aerosol insecticides will do a good job of killing the spiders. Before using, read the label for precautions and application sites. Note: application to plants may cause severe burning.

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.

Monday, December 5, 2011


Should I scrape off the light colored mossy growth that is growing on the trunk of my citrus tree?
 Lichen is actually two different organisms, a fungus and an alga living together. It is the fungus that provides the shape with the algal cells growing in a layer near the top of the fungal structure. The algae manufacture food, a process called photosynthesis, for both the fungus and the algae: a symbiotic relationship.
Lichen is frequently seen growing on twigs and branches, especially under moist conditions.  The lichen structure is usually grayish-green in color, paper-like, and forms small circular to irregular-shaped spots.
When lichen is wet from rain or dew it grows actively, but when dry, it stops growing. It doesn’t die but lies dormant until the next rain starts it growing again.

 Lichens are considered harmless.  Although they attach themselves to the tree, they are not pulling vital nutrients from that tree. It is often thought that lichens are injuring a tree because they are growing so well, and the tree looks so bad. This is just an indicator that some other requirements of the plant are not being met.  Check and make sure there is no insect infestations (scale, aphids), no old weed- whacker damage and make sure the tree is adequately fertilized. 

It has been found that most lichens will not grow in a smoky or polluted atmosphere.

Friday, December 2, 2011

What Factors Determine Sweetness in Citrus Fruit?

The accumulation of heat units is most important in determining the sugar content in the fruit.  Simply put, citrus fruit become sweeter the longer they remain on the tree (to a point). If you think the fruit is ready to pick but find that it’s not sweet enough, leave it on the tree a little longer; it should sweeten up.   
What if it doesn’t increase in sweetness? Of the hundreds of citrus varieties, not all are adapted to one specific area. For a given region, some varieties just won’t produce good quality fruit.
The rootstock factor -  There are numerous citrus rootstocks.  They impart various different quality characteristics onto the fruit.  Some rootstocks will enable the tree to produce abundance of fruit, but with poor fruit quality (low sugar content).  In general, homeowners do not have much choice in rootstocks when purchasing a tree. Commercially, it is an important factor when selecting new trees for an orchard.