- Author: Jeannette Warnert
UC Agriculture and Natural Resources scientists based at UC Riverside are honing in on odors that might lure Asian citrus psyllids into traps, and other odors that will keep them away from citrus trees, reported Mark Muckenfuss in the Riverside Press-Enterprise.
Anandasankar Ray, professor in the Department of Entomology at UCR, along with two other researchers, published results recently that Ray believes are promising enough they may soon be adapted for grower use.
Ray and his team tested three attractant odors in El Monte backyards using yellow sticky traps. More than twice the number of psyllids were found in the scented traps compared to unscented traps, the article said. In time the researchers will also test chemicals that can mask odors that are pleasant to Asian citrus psyllids and some that repel the insects.
Other research projects underway at UC Riverside to combat Asian citrus psyllid and the disease it can spread were also noted in the Press-Enterprise article. They are: biological controls, including a tiny wasp imported from Pakistan that feeds on the psyllids; insecticides; developing resistant strains of citrus trees; finding a way to kill the bacteria spread by psyllids once it is in the tree; and discovering ways to identify diseased trees earlier.
Edema may be caused by any agent that stimulates an abnormal increase in the size and number of a group of inner cells. Edema can be induced by (1) spraying with some chemicals such as ammoniacal copper carbonate in an oil emulsion, (2) injuries resulting from wind-blown sand particles and sucking insects, (3) high light intensity (over 2000 foot-candles) for ivy geraniums, and (4) accumulation of water in the intercellular spaces.
The most common cause of edema is the presence of abundant, warm soil water and a cool, moist atmosphere. Under these conditions the roots absorb water at a rate faster than is lost through transpiration. Excess water accumulates in the leaf, some parenchyma cells enlarge and block the stomatal openings through which water vapor is normally released from the plant; thereby contributing to further water retention in the leaf. If this condition persists, the enlarged cells divide, differentiate a cork cambium, and develop elongate cork cells externally to form a periderm. The rupture of the epidermis by the enlarged inner cells and the periderm account for the raised, crusty appearance of older edema spots.
Changes in weather and cultural practices of growing plants usually will avoid edema. To reduce the risk of edema occurrence, the following may be helpful:
1. Avoid irrigation or watering during cool, overcast humid weather. For potted plants in greenhouses, remove saucers under pots, or discard any water that remains in the saucer 30 minutes after watering. Irrigate or water when air temperature are rising or humidity is low.
2. In greenhouses: a) reduce the humidity of the air by venting and increasing heat; b) improve air circulation; c) increase light intensity; d) space the plants farther apart; e) for potted plants use a well-drained potting medium for potted plants and avoid standing water in saucers under the pots.
3. Avoid overfertilizing, especially when the plants are growing slowly, such as during the winter months. Maintain fertility based on a soil test. Avoid low levels of potassium and calcium.
4. Avoid cultivars that are highly susceptible to edema under your growing conditions.
The black spots on the fruit start out as a clear exudate. The same for the brown spots on the leaves.
Impacts of the recent drought conditions on Central Coast avocado production, and potential impacts of continued drought conditions
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction. Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Defoliation leads to sunburned trees and fruit which can be severe economic losses.
Strategies to address drought conditions
Ensure that the irrigation system is at its greatest potential and is maintained. Avocados are grown on hillsides and pressure regulation is extremely important and is frequently neglected.
Significantly prune trees to reduce leaf area. Avocado can be a very large tree, and if half the canopy is removed, there can be as much as 1/3 reduction in water use. When trees are about 15 feet tall, removing half the canopy can reduce water use by one half.
In extreme drought conditions, the canopy can be reduced to just the skeleton branches which are white washed to prevent sunburn. Water use drops to zero, and then gradually as the tree leafs out, water can be slowly reapplied, but at significantly less amounts than with the full canopy. Stumping typically results in three years' worth of crop.
In orchards that have low producing areas, because of recurrent frost, high winds, shallow soils, disease, etc. the grower could decide to completely remove those trees, thereby saving water.
White kaolin applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since if it is applied to fruit, it is very difficult to remove.
Impacts of the recent drought conditions on Central Coast citrus production, and potential impacts of continued drought conditions
Citrus is much less sensitive to salts and drought than avocado, partially because of its greater rooting depth. However, it is much more sensitive than deciduous fruit trees, resulting in smaller fruit and lower prices when drought cannot be addressed with adequate irrigation water. Drought also makes the trees more susceptible to leaf drop, and sunburned fruit.
Strategies to address drought conditions
The strategies for citrus are very similar to those for avocado. It is much more sensitive to pruning to reduce water use than avocado. Typically removing half the canopy results in half the water use. Because of thus greater control, citrus is rarely stumped.
By reducing canopy size, production can be maintained, often without loss of fruit size.
Kaolin clay can effectively reduced water use and can be applied soon after harvest without the problem of coating fruit making its removal difficult at the packing house.
This is a story about phone calls that come in to my message machine. Yesterday I got 3 calls from PCAs (Pest Control Advisors) and two from growers. The two from growers were from a Papaya grower and the other from a dragon fruit grower that I am working with to develop an industry here in Santa Barbara/Ventura. One is in Carp the other in Montecito. The PCA calls were from two that work on avocado and the other from a citrus grower. I either get a call from an avocado grower direct because they can't afford a PCA or from the manager or the PCA of larger farms. From citrus it is usually the manager or the PCA. These are more developed industries and the grower usually lets the workers take care of problems because they are so familiar with the operations. When it ‘s a new crop, the owner steps in. They want to know all that can go wrong with this new crop.
Blueberries are expanding now along the coast and when we first started working on them 15 years ago, our collaborators were in touch with us constantly. Now it's the managers who call. Its now a developed industry. The same for coffee. When we started working on it we worked closely with out cooperators, now there is a coffee cooperative that takes care of itself. We work with new things. One of the calls from an avocado PCA was about a farm that is being infested with bagrada bug. Everything in the area has dried up from the drought. The bagarada bug normally goes after plants in the brassica family (cabbage, mustard, etc.), many of which are native and growing along streams and on hillsides. The streams and hills have dried up and the bug is now going to the new avocado leaf tissue and the PCA wanted the bug identified and to provide a solution to the problem.
Sometimes the weather works for us and sometimes against us. We've got a parasitic wasp for controlling olive fruit fly. We went out all over southern Santa Barabara county to groves of olive trees to determine where to release the wasp and could find healthy olives, but no olive fruit. No where to make to make releases this year. The weather plays tricks on us. But we keep looking for solutions. Maybe next year we will be able to study if and how the wasp controls olive fruit fly. It does in France.
- Author: Neil O'Connell
Since the first publication of this article on vole damage in citrus, a recent field observation regarding vole activity is worth noting. Voles prefer a situation where there is cover and shelter generally from weed or grass. In some orchards established in the last few years a plastic strip has been installed along the tree row for weed management (Fig 1). These strips appear to be offering a sheltered environment for vole activity in some cases (Fig 2). Recent observations in two such installations, one a block planted in 2013 and the other an eight year old planting exhibited significant vole activity. The young orchard at this point does not exhibit obvious tree damage although active tunneling is apparent (Fig 3). In the older orchard feeding damage to the trunks is very obvious (Fig 4).
Meadow Mice (Voles) can cause serious damage in a citrus orchard resulting in partial or complete girdling of a tree (Fig 5). Trees often exhibit damage to the bark of the tree from the soil line up 6-8 inches (Fig. 6 ). On close inspection, an open hole 1-1.5 inches in diameter may be found at the base of the tree (Fig 7).
Five species belonging to the genus Microtus are found in California, two of which “Microtus californicus” and “M.montanus” are reported to cause damage. Damage has been reported in permanent pasture, alfalfa, hay, artichokes, Brussels sprouts, carrots, cauliflower, potatoes, sugar beets, tomatoes, grains, nursery stock and the bark of apple, avocado, citrus, cherry and olive trees.
Microtus are often found where there is grass cover. They generally do not invade cultivated crops until the crop is tall enough to provide food and shelter. Meadow mice are active all year round. They forage at any time during the day or night but are chiefly nocturnal. They are usually found in colonies marked by numerous 2-- inch wide surface runways though matted grass. Small brownish fecal pellets and short pieces of grass stems along the runways are evidence of activity. The burrows consist of extensive underground tunnels, nest chambers and storage chambers. Home range is typically small, less than a 60 foot radius in the case of “M.californicus”. All meadow mice swim well. Therefore, irrigation ditches will not serve as effective barriers against meadow mice movement into fields. Meadow mice may forage beyond the sheltered runways. Food consists of tubers, roots, seeds, grain, and succulent stems and leaves.
Females breed at 4 to 6 weeks of age with litter size of “M.californicus” averaging around 4. Under natural conditions a female Microtus may produce from 5 to 10 litters a year. The major breeding season corresponds with the season of forage growth. Microtus populations build up to a peak every 3 to 4 years, followed by a rapid decline during the next breeding season. The exact causes of the cycle of buildup and decline are not known, though disease, food shortages, physiological stress from overcrowding, and other factors may be involved. It is assumed that in cultivated areas Microtus populations are permanently based in favorable habitat such as roadsides, canal banks or adjacent noncultivated land. Invasion of cultivated cropland occurs when the population builds up or when the wild habitat becomes unfavorable. Coyotes, badgers, weasels, snakes, hawks, owls, herons and gulls are among the principal predators. It is believed that predators are not able to prevent or control a population eruption because of the birth rate of the fast breeding Microtus population. Meadow mice are classified as nongame mammals by the California Fish and Game Code. Nongame mammals, which are found to be injuring growing crops may be taken at any time or in any manner by the owner/management. The most effective management options in an orchard situation are a reduction in ground cover and the use of toxic baits. Meadow mice are cover dependent. In situations where cover removal is not possible or is insufficient to solve the problem, the next best option is the use of toxic baits. Many bait carriers are used (e.g., oat groats, wheat bait). Baits: Crimped oat groats are the most satisfactory bait although crimped whole oats are used (e.g., oat groats, wheat grains, pelletized formulations, etc., but crimped oat groats have typically been most effective). The primary toxicants used for meadow mouse control include zinc phosphide, diphacinone, and chlorophacinone. Directions for management including baiting can be obtained by contacting the Agricultural Commissioner's Office. * Portions taken from J.P.Clark Vertebrate Pest Control