Friday, November 6, 2020

Avocado trees - Spots on the leaves?





Originating in Mexico, the persea mite, Oligonychus perseae, was found in California in the early 1990’s and probably came to the Big Island of Hawaii in the early 2000’s. The main host for the mites is avocado trees. Some varieties are more susceptible than others; the Lamb-Haas is considered least susceptible. Many ornamentals and weeds also host the persea mite.  The adult mites have eight legs and an oval body with two dark reddish eye spots near the head end of the body. They lack antennae and body segmentation.


                                                            

Mites feed both on the fruit and leaves, most notably on the underside of leaves. The damage appears as small, circular, yellow to brown spots. Mite colonies often reside against the protruding midrib vein of the underside of the leaves. They produce a dense, silk webbing of protection over them. The webbing resembles a silvery spot on the underside of the leaf, often seen glistening under sunlight.  


Premature leaf drop will begin once the feeding damage to the leaves reaches and exceeds 10% of the surface area. This can lead to sunburned bark and fruit; yields have been reduced up to 20 %.  Trees will recover from the defoliation by producing a new flush. But repeated dropping of the foliage will severely stress the tree.


In the home garden, some mites can be tolerated. Several species of natural enemies often reduce the population of the mites. If numbers get too high, horticultural oils will suppress the mite population. It is important that the spray solution contact the undersides of the leaves, where mites are located. Where feasible, spraying the undersides with a forceful stream of water can also reduce mite populations. By using hydrated lime or diluted white latex paint, a whitewashing on the trunk and major limbs will help protect bark and wood from sunburn after a premature leaf drop.

Photo of Mites: UC IPM Program

Friday, September 25, 2020

Pickleworm

                                                           
  
Why does pumpkin squash ( kabocha/kombucha, Japanese pumpkin) grow into a nice vine, but many of the young fruit turn yellow and drop off? 

This sounds like a case of an insect pest called the pickleworm( Diaphania nitidalis).  This moth was discovered in O`ahu in 2003, and by 2005 had spread to Kauai, Maui and the Big Island.  It attacks many cucurbits such as cucumber, squash, pumpkin, zucchini and cantaloupe.  Squash seems to be the most susceptible. The caterpillars like to feed on the blossoms and afterward will burrow into the developing fruit. The adult moths are colored with yellow and brown and have a purplish sheen. 


Some insecticides are registered on cucurbits and have been successful, with diligent spraying, in commercial crops.  One organic insecticides that may be effective on the pickleworm is known as Bt, (Bacillus thuringiensis).  It is actually a bacterium which is deadly to the moth larvae, but not to humans.  One problem is coverage; the spray adheres to the surface of the leaves and blooms, while the caterpillar is on the inside of the bloom and young fruit.

                                                              

It has been difficult for the homeowner to control the pickleworm. One method is exclusion.  Place screening material over the plants, or individual blooms.  This will keep the moth out and prevent her from laying eggs.  The problem however, is that the cover would also exclude honeybees.  Honeybees are important for pollinating many of the cucurbit crops.  A lack of pollination can be a cause of blossom drop.  Because the moth is a night flyer, you can cover the crop at dusk and uncover it during the day.  This will allow honeybees to visit the plant during the daylight hours, but keep the moths out at night.  Admittedly this is quite a tedious process.

Alternatively, since the moth is relatively large, a wingspread of about one inch,  ¾ inch bird netting can be used to build a permanent structure over the plants. This will exclude the moth while allowing bees to freely move through the netting.

There are a few squash varieties which show some resistance to the moth.  They are Butternut 23, Summer Crookneck, Early Prolific Straightneck and Early Yellow Summer Crookneck.  There have also been reports from other areas of the country, that the early crops have less damage (may not be applicable to Hawaii).

Recently a company named High Mowing Organic Seeds has released a new variety of zucchini called Partenon.  It is a parthenocarpic variety which means that fruit develops without pollination. Thus this variety can be grown in places that  exclude pollinators (honeybees) such as a greenhouse or netted structure.   Partenon is also tolerant to powdery mildew.  (www.highmowingseeds.com )

Monday, August 24, 2020

The Corn Earworm



The corn earworm is one of the most destructive insect pests of corn in the world. But here in Hawaii it is not quite as destructive thanks to CTAHR (University of Hawaii College of Tropical Agriculture and Human Resources) plant breeders. Hawaii-bred corn varieties are more resistant to the earworm due to their thicker and tighter husks, thus limiting the number of larvae that can gain entry to the ear. This allows growers to simply cut off the tips of the ears to remove any damage that may have occurred.

Most of the eggs of the corn earworm are laid on the young silks soon after the silks have emerged. Young larvae crawl down the silk to feed on the kernels, soft cobs and the silk itself.  Luckily they also eat each other, keeping populations low.

Corn planted early in the year is not as seriously affected as is late corn because population densities increase as the season progresses. Early plantings will have minimal damage; later in the year one or two worms may appear in the tops of each ear of corn.

Control
To help control the pest, between plantings destroy the crop residue or haul it off to the compost bin. This eliminates places that would harbor the pest. Several natural enemies are present in Hawaii and, in general, they keep the corn earworm at tolerable levels. In most cases, control is simply a matter of cutting off damaged ends of corn at harvest.

For those gardeners who are plagued with the corn earworm, here are some organic  pesticide recommendations from the University of California IPM program:
  • Spinosad -  must be applied on silks within 3 days after first silks appear and at 3-day intervals until silks turn brown.
  • Bacillus thuringiensis (Bt) -  may be dusted on silks every 3 days after 5 to 10% silk formation for partial control.
  • Applying a few drops of mineral oil with a medicine dropper to silks just inside each ear 3 to 5 days after silks first appear may be effective.