Monday, August 19, 2013

White Deposits on Avocado Trunks

What causes the white exudate on avocado trunk and branches?

When avocado trees are wounded, tree sap oozes out. The white residue seen on tree trunks is simply the sugars left behind as the water from the sap evaporates. The wounds can be made by just about anything including poking holes with a knife or nail and insects trying to bore into the tree. 

There are also a few avocado diseases, caused by fungi and bacteria, which will result in wounds that ooze leaving a white residue. In areas of heavy rainfall the sugars are not always visible as they may be washed away. 

Although not a major pest of avocados, one invader which will leave small amounts of white residue is the black twigborer.

Search black twigborer on this website for more information on this pest.

Photos from Topics in Subtropics,by  Dr. Ben Faber

Monday, August 12, 2013


Gladiolus is relatively easy to grow.  They, gladioli, require well-drained soils in a sunny location. While often classified as bulbs, gladiolus are actually corms, enlarged stems. Corms which have a plump shape with high centers are better than large, flat ones; the thicker the corm is, the higher the quality of bloom will be.  It is best to select corms 1 1/2 to 2 inches in diameter. They will produce full, attractive flower spikes. Small corms produce foliage but may not bloom.

Planting depth varies with the size of the corms. Large corms should be planted 4 to 6 inches deep and 6 inches apart. Small corms should be planted at a depth of 3 inches and about 2 inches apart.   A general rule of thumb is plant deeper in sandy soils and shallower in heavy soils.

Gladiolus may fall over because of the weight of the blossoms if the corm is not planted deep enough. The remedy, of course, is to plant deeper for better support.  If the soil is rocky, and it is difficult to dig a deep enough hole, the plants will require staking. 

The pest that primarily attacks gladioli are called thrips. They are tiny, slender black insects, about 1/25 of an inch long, obviously, difficult to see with the naked eye.  Thrips cause white specks or streaking on the leaves and blooms. Flowers can also be misshapen, discolored and occasionally will fail to open. 

Thrips feed by puncturing the plant tissue and sucking out the cell contents.  Preferring to feed on rapidly growing tissue, thrips often cause stunting.  These insects are the culprits which also cause silvery to brownish scarring on avocado and citrus fruit surface, but this cosmetic damage does not harm the internal fruit quality.

Horticultural oils, azadirachtin (Safer BioNeem), neem oil and insecticidal soaps can be somewhat effective for the temporary reduction of the number of thrips if applied when damage first appears. Sprays must be applied to thoroughly cover susceptible areas of the plant including new leaf growth and buds.  Note: these insecticides only kill newly hatched thrips and recently emerged adults, thus repeat applications are required, usually 5 to 10 days apart depending on the temperature. Moving the location of the gladiolus bed each year will discourage the spread of thrips.

Avoid the use of insecticides such as malathion and carbaryl: 1) they are highly toxic to natural enemies, 2) can cause dramatic increases in spider mite populations, and 3) are generally not effective against most thrips.


Monday, August 5, 2013

Fertilizers - Nourishing the Garden

Which is the best fertilizer? With the countless number of fertilizer products on the market, little research has been conducted to show whether there is significant differences among the various brands. If you have found a particular product that produces outstanding results, and you are willing to pay the price, then by all means, continue to use it.

Otherwise, let me give you some fertilizer basics. In the broadest sense, nitrogen is nitrogen (N), phosphorus is phosphorus (P), and potassium is potassium (K).  Plants only absorb nutrients that have been broken down into inorganic, water-soluble forms.  For example, whether nitrogen comes from brand A or B, whether it was derived from anhydrous ammonia (gas) or urea, or from manure or the compost pile, it makes no difference to the plant. Stick with a basic N-P-K fertilizer, buy on sale, and don’t fall for promotions.

The question now is what type of formulation to purchase?  Nitrogen only, potassium only, phosphorus only, or combinations?   Nitrogen is always needed in areas of heavy rainfall, because it is easily leeched out of the soil.  Fertilizers like ammonium sulfate, ammonium nitrate, urea and calcium nitrate are all good fertilizers for supplying nitrogen. Calcium nitrate will also give calcium but is more expensive. 

Like nitrogen, potassium can also move in the soil with water; phosphorus moves very slowly. The question of whether to apply potassium and phosphorus can be answered accurately by a soil analysis.  Without an analysis, statements about soil fertility are only guess work. Check with local university cooperative extension offices for information about soil analysis.

 If an analysis is not taken, then buying a fertilizer with all three ingredients (N-P-K) is a good idea. The only problem with that is if the soil has adequate phosphorus, and over the years more is added, a soil imbalance may occur, and the excess phosphorus will cause a deficiency in iron and zinc - seen as interveinal chlorosis, a yellowing between the veins.  Also, it is cheaper to supply nitrogen than all three nutrients. On the other hand, you can apply nitrogen alone and supply the potassium and/or phosphorus only when deficiency symptoms occur.

Sometimes micronutrients are added to an N-P-K formulation. But they are added at such low amounts that they are seldom beneficial to the plant, especially trees. If a micronutrient (zinc, manganese, magnesium) deficiency truly occurs, buy that specific nutrient and apply to the soil or as a foliar treatment, whichever is appropriate. Special formulations of ‘citrus food’ or ‘avocado food’ or ‘palm food’ are good fertilizers. But note that they are higher priced, and a basic all-purpose (N-P-K) fertilizer should work as well.

Organic vs. Inorganic
Inorganic fertilizers often contain rather high percentages of the major nutrients and are applied on a per pound per acre basis. Organic fertilizers, on the other hand, have relatively low amounts of nutrients and are applied on a per ton per acre basis.  Although organic fertilizers have many benefits - including improving soil structure and the water holding capacity of the soil, feeding beneficial microorganisms and  of course, acting as a reservoir of nutrients that can be released to the soil - they are rather expensive in some places like Hawaii.  Gardeners, however, are encouraged to take advantage of any free or inexpensive supply of organic fertilizers, i.e., manures, in their area.