Black Sooty Mold

What is the black, sticky substance that may be found covering the leaves of many plants?

The black substance on the leaves is called sooty mold.  It is a fungus which resembles soot. This particular fungus, however, is not harmful to the plant but is actually living on a sweet, sugary substance called honeydew.  The honeydew is being secreted by some insect that is infesting the plant. If you see sooty mold on a plant, it means that the plant has an insect infestation – most likely aphids, mealy bugs, soft scale or whitefly. 

As the insect feeds, a clear sugary liquid is secreted by the insect onto the leaf below.  It is on this secretion that the mold grows.  Heavy rains will wash off the sooty mold from the leaves, but the insect problem still needs attention. Often when large trees become heavily infested with certain insects, the honeydew can actually be seen raining down upon the ground.  Again, sooty mold indicates an insect problem; inspect the plant for bugs.  Below is a close up of the sooty mold.

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

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 )

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.

Bugs - the Good, the Bad and the Ugly

Not all bugs in the garden are bad. 
Some bugs are beneficial. such as parasites and predators. The are looking for pests to consume. Two well-known good guys are preying mantis and lady bird beetles; spiders are also good predators.  Biological control can work very well. As an example, in California when certain species of whitefly came into a new area, both citrus  and some vegetable crops were damaged or completely destroyed. Yet within a number of years, their natural enemies were established, and for the most part, the pest became inconsequential.  Pollinating insect can also be categorized as beneficial.

Some bugs may be found on a plant, but are not feeding and therefore cause no damage. The bug may simply be resting or perhaps searching for nectar or another source of food.                                                                                                   

               
Some insects of course are known pests, but cause little damage. A bite may be taken from a leaf here and there,  or a little juice sucked from a stem, but overall, the plant is not harmed. 
On the other hand, some bugs do inflict damage, but it is only cosmetic. That is, the leaves or fruit may be scared or misshapen, but there is no real loss of yield.  In many cases, cosmetic damage should be tolerated. This type of damage on citrus fruit is caused by thrips and does not affect the internal quality.    

Above Photos: University of California IPM Program

And finally, if there is an infestation of an insect pest which will cause yields to be significantly lowered, the gardener will have to decide whether to wait a bit and see if biological control will work. If a parasite or predator can control the pest population, no intervention is needed. Yet if spraying is the choice, consider first a bio-rational pesticide, such as soap, oil, Bt or sulfur. These are less injurious to the environment, and may not harm the predator or parasite populations. 

And, the Ugly

Arizona State University

Trouble with Palms

"The bottom fronds of our triangle palm started to die, and then suddenly the whole palm fell over," a friend complained.

A significant pest of Pritchardia palms is the banana moth, Opogona sacchari. The triangle palm is also attacked by this moth. It seems that the female moth targets wounded or stressed palm tissue to lay her eggs. Stresses can include drought, flooding, mechanical wounding, poor nutrition, and herbicide injury. The larvae generally feed on decaying and dead plant tissue but will feed on living tissue, too, causing extensive damage. In affected palms, larval tunneling, along with the characteristic frass (insect droppings), can often be seen. Fully developed caterpillars measure just over an inch. The adults have greyish brown wings are 3/8 to 5/8 inches long.

The main treatment is prevention: keep palms growing well; give them adequate fertilizer, and supplemental water during a drought. Gardeners also need to be careful with the weed wacker! Female moths are looking for wounds to lay their eggs. Bt (Bacillus thuringiensis), an organic insecticide, can be applied to these wounds, as well as those caused during pruning.

For more information about this pest see the CTAHR Cooperative Extension Service publication, “Banana Moth – A Potentially Fatal Pest of Pritchardia and Other Palms”, by Scot Nelson and Mark Wright.  CTAHR stands for the College of Tropical Agriculture and Human Resources at the University of Hawaii. 

Black Witch Moth

"May I borrow your house?" is the Mayan translations for Mah-Ha-Na. This is the name for the moth which often rests at night under the eaves of the house. The common name is Black Witch Moth, Ascalapha  odorata.

Its wingspan can reach 7 inches.  The wings are dark brown, and both pairs are crossed by a series of alternating light and dark wavy lines. There is often an iridescent blue cast over the wings. Females have pinkish-white bands across the middle of both wings, whereas the males lack these pale bands.  In addition to the Hawaiian Islands, they are common in the Caribbean, South and Central America and migrating into the continental USA and southern Canada in the summer.  

This moth lives in the tropical and subtropical forests where trees of the pea family grow. This includes acacias, albizia, cassia and samanea (monkeypod). The caterpillars feed on the foliage of these trees. The moth often flies great distances in only a few nights, hiding by day wherever it can find dense shade, frequently under the eaves of houses.

While they mostly fly during the summer season, in the southern areas of the United States and in Hawaii, they are also known to fly during late October, hence the name Black Witch Moth. 

Mealybugs and Aphids on the Roots

Pineapple mealybug             

 There are species of aphids and mealybugs that attack the roots and the crown line of plants. Many times when plants are pulled out of the pot, whitish fuzzy bugs in the soil or on the roots can be seen; they are the root aphids or root mealybugs.

In Hawaii, the pineapple mealybug is common. The adult females are described as plump and convex in body shape and pinkish in body color. Primarily a pest of pineapple and other bromeliads, yet these mealybugs also attack banana, celery, citrus, cotton, coffee and hibiscus. Wherever pineapple is grown, the pest is present including all of the major Hawaiian Islands. Plants rarely die from a mealybug infestation.

Control of this pest depends on controlling the ants which provide the mealybugs shelter and protection from predators and parasites. Without ants, mealybug populations are small and are slow to invade new areas because of the presence of the many natural enemies (the good guys) that are in Hawaii.

In infested areas of the garden, after the crop is removed, the soil should be turned over and all crop residue removed including grassy weeds which may harbor the mealybugs.  Host plants of this pest (listed above) should not be grown in this bed for at least a year.