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.
I was recently in an avocado orchard and saw the rounded fruit and small leaves typical of zinc deficient trees. I asked the grower if there were recent leaf analysis of the orchard, and so we looked at them. The leaves were running at 20 ppm which is low. Sufficiency runs at 50 ppm. The recommendation was to apply zinc sulfate to the soil. The recommendation included, though 200 pounds of phosphorus per acre. Phosphorus and zinc are antagonistic, meaning applying one can limit uptake of the other. In applying phosphorus at such a high rate was probably preventing uptake of zinc. It is also antagonistic to copper, iron and manganese, so all of these micronutrients can be limited by phosphorus applications.
There have only been two documented cases of phosphorus deficiency in fruit trees, walnuts on a volcanic soil in Lake County and oranges on decomposed granite in San Diego. It is an essential element, yes, but applying it when there is sufficiency in the leaves can lead to other problems which can be hard to correct. Generally speaking, phosphorus does not need to be applied to fruit trees in California. In other states that have peat soils, high carbonates or highly weathered soils, phosphorus application is a normal practice, but here make sure you need it before applying it.
By way of the San Francisco Chronicle
Hopes of an almighty El Niño bringing rain to a drought-stricken California - with its fallow fields, depleted streams and parched lawns - were further dashed Thursday. The National Weather Service, in its monthly El Niño report, again downgraded the chances of the influential weather pattern occurring in the fall or winter.
The odds were 80 percent in May, but were placed between 60 and 65 percent this week.
Meanwhile, the agency also announced that the much-needed weather event is likely to be weak instead of moderate in strength - another retreat from the more robust projections made earlier this year that fueled speculation that California's three-year dry spell might be snapped.
Just when you thought we knew all the devastating pests of avocado, along comes another one, this showing up in the Piru area of Ventura County - bagrada bug. The infestation is in huge numbers covering the stems and leaves of the tree, leaving fecal droppings on the backsides of leaves. The insects have caused some defoliation and damage to terminals. They lay their eggs in crevices in the soil and sometimes on the plant. Maybe mulching would help in their control. The orchard will be sprayed soon to control them.
The bagrada bug, Bagrada hilaris, also called the painted bug, is a stink bug that attacks various vegetable crops, weedy mustards and several vegetable crops like cabbage, broccoli and cauliflower. It is particularly devastating to young seedlings and leafy mustard greens.
Bagrada bugs often infest wild mustard weeds, which are pervasive in California on hillsides and in agricultural corridors in late winter to early spring. Populations rapidly increase in the weeds when seasonal temperatures rise. Record numbers of bugs can invade newly planted cole crops after mustard weeds dry out in late summer.
The bagrada bug is an invasive pest species, native to Africa, which has spread to India, Pakistan, parts of Southeast Asia, and Italy. In the United States, it was first found in Los Angeles County in 2008. By 2011, the pest had disseminated throughout Southern California to include San Diego, Imperial, Orange, Riverside, San Bernardino, and Ventura counties. In September 2012, the pest moved northward to Santa Barbara and San Luis Obispo counties and recently (2013) the Bagrada bug was found in Fresno, Tulare, and Monterey counties.
In this case, it appears that with the drought all the mustard on the hillsides has dried up and the only food is the young avocados on the hillside. Interestingly, the neighboring canyon is infested with harlequin bug (Murgantia histronica) another stink bug with similar coloration but about three times larger than the bagrada bug. Once rains return, these pests should retreat back to the wild hillsides.
Images: 1) Dry hillside where infestation is occurring. 2) Defoliation caused by bug. 3) Fecal pellets on underside of leaves. 4) Bagrada adults and nymphs.