Monday, May 25, 2015

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

Monday, May 18, 2015

Poor Seed Germination - Nothing's coming up!





Planting a vegetable garden has a variety of challenges. But how disappointing when the seeds are planned and nothing comes up! Here are primary reasons for poor seed germination.

 1. Planting depth – small seeded vegetables like carrots and onions can easily be planted too deep, and if planted too close to the surface, can easily be washed away during heavy rains. Consult the seed packet for proper planting depths. 

2. Temperature – some seed need the soil temperature to warm up before they will adequately germinate, while other cool season vegetables like onions, germinate best in a cool soil. 

 3. Water, of course, is important; too little and they will not germinate or dry up after germination. With too much water, seeds and seedlings can die from lack of oxygen or be overcome by fungi. Even under normal moisture conditions, different species of fungi and bacteria can cause seeds to rot.  Seeds and seedling should always be kept moist but the soil should not remain soaking wet.  

 4. Soil pH is also important. Each vegetable seed as well as other plants will have its range in which it will germinate best. 

5. Herbicide residues may still be active in the soils causing poor germination.  A high fertilizer content in the soil could also hamper germination. 

6.  Insects will often feed on  seeds and young seedlings.

7. And finally, the seed itself! Poor seed germination can particularly be a problem in humid climates.  Relative humidity influences the moisture content of the seed. The higher the moisture content, the lower the germination rate will be, especially, after one year.


As a general rule, vegetable and flower seeds can be kept for one year without appreciable decrease in germination. Storage, however, may be extended to 10 or more years under proper conditions; seed moisture and temperature are the most important factors. The drier the seeds, the longer they will store.


In humid climates such as Hawaii, storing vegetable seeds in a dry, sealed glass container should keep most seeds viable for a year. For longer storage, place seeds in a moisture-proof container and store in the refrigerator.


Longevity of vegetable seeds will also vary depending on the species.  For example, collards, cucumber, endive, radish and water cress produce some of the hard seeds, rated as lasting 5 years. Next come beets, cabbage, cauliflower, eggplant, kale, mustard, squash, swiss chard, tomato, turnip, watermelon, rated at a 4 year longevity; asparagus, beans,  carrot,  celery,  chinese cabbage, New Zealand spinach, pea, and spinach, at 3 years; leek,  sweet corn, okra and pepper are rated at 2 years; and finally, the last group is rated at a mere 1 year longevity - lettuce, parsley, parsnip and onion.



Photo: Purdue University

Monday, May 4, 2015

Weeds in the Lawn



Controlling weeds should begin long before chemical herbicide applications are considered. 
Cultural measures ought to be employed from the beginning. These include 
a. proper irrigation -  weeds often invade lawns that are either overwatered or underwatered. Perhaps the worst possible irrigation schedule, yet common, is daily watering for 5–10 minutes. Light, frequent irrigation creates a shallow-rooted lawn which is more susceptible to weed invasion, as well as being less tolerant to drought.  In dry areas, where irrigation is employed, water should penetrate to a depth of 6 to 8 inches. The best practice is to allow the soil to partially dry out between irrigation. The top 1 to 2 inches  should be fairly dry before watering again. For more information on irrigating lawns, in Hawaii see UH CTAHR publication, “Watering Lawns”, at http://www.ctahr.hawaii.edu/site/info.aspx;  or your local University Extension office or website.

b. Proper fertilization – apply suitable amounts of fertilizer on a regular basis; nitrogen is the key nutrient for turf.

c. Proper mowing heights – each turfgrass species has an appropriate mowing height. Mowing some grasses too short can weaken the lawn and predispose it to weed invasion.  A typical guide is to remove no more than one-third of the leaf blade at each mowing. If too much is removed, it will take more time for the grass to recover, giving weeds a chance to invade.

d. Thatch is a layer of organic matter - stems, stolons, roots - that develops between the turfgrass blades and the soil surface. Regular thatch removal will help keep turfgrass healthy and competitive with weeds.  See the UH CTAHR publication “Removing Thatch from your Lawn” for more information on this procedure.

e. Any activities that lead to soil compaction will contribute significantly to turfgrass stress. This in turn, will make it easier for weeds to invade. These activities may include heavy foot traffic as well as vehicles traveling over the area.

It is impossible to get 100% weed control in your lawn. Weeds will occur, but the problems can be minimized with a well-managed, vigorously growing lawn. By combining cultural methods with herbicide applications when needed, weeds will  be minimized.

When controlling weeds in your lawn it's good to know the name of that weed to see if it is listed on the particular herbicide you have chosen to use. But what's more important, before identifying the species, is to know the type of weed: broadleaf or grassy weed.  Distinguishing between the two will determine the appropriate herbicide to use.  Controlling a grassy weed in a lawn (grass) is difficult since most chemicals cannot distinguish between good and bad grass species.

Fortunately, certain herbicides can distinguish between grassy plants and broadleaves. It is the chemical composition of the herbicide and the morphology of the plant that will cause certain herbicides to affect broadleaf plants but not affect grasses. Therefore, controlling broadleaf weeds in a grassy environment is relatively easy. Purchase an herbicide with the active ingredient 2,4-D, MCPP (mecoprop), or dicamba (Banvel). These products are often sold in combination, giving better management and a wider spectrum of the broadleaf weeds controlled.

For light weed infestations, spot treating is more appropriate than treating the entire lawn. Apply just enough of the solution to wet the leaf, do not apply to the point that the herbicide is dripping off the leaf. To insure maximum absorption, stop mowing 2 or 3 days before treatment and allow 3 or 4 days before mowing again. This allows sufficient time for the weeds to absorb the herbicide and transport it to their roots.
Caution: these materials are volatile, especially 2,4-D. The drift can damage other valued broadleaf plants; tomatoes and hibiscus are particularly sensitive.

Photo by Forest and Kim Starr - Nene, the Hawaiian goose, feeding on weeds and grass in a lawn setting.
 Weeder geese are used with great success to control and eradicate troublesome grass and certain weeds in a variety of crops such as corn, flowers, strawberries and grapes. Note: Geese prefer grass species and will eat other weeds and crops only after the grasses are gone and they become hungry.