Soil likes to be covered at all times.  It doesn't “want” to be exposed to the elements, so you either cover it (plants, asphalt, paper etc.) or it will cover it for you with plants (weeds).  If it can't be covered fast enough, it disappears – erodes.  This can be from wind or rain or just natural movement down slope.  Plants that are managed for other than their agricultural return are called cover crops, although they can also have a crop that is saleable. Often weeds can be managed to be a cover crop, as well.

We are looking at a possibly wet winter and many tree crops grown on hillsides and sloping ground are prone to soil erosion.  Covers can be grown year round, but that usually means they require water all year round.  That means they need an irrigation system dedicated to their needs.  It also means having extra water which may be limiting.

A winter cover crop that grows out in the winter, does its thing (although that “thing” can include lots of other things, e.g. insectary, nutrients, water retention, etc.), and then dies or goes dormant, can be ideal.  It also requires less water than a permanent cover.

But there is a big problem here. Establishment of an introduced cover still requires water.  Rainfall in Southern California is erratic and there may be early rains to germinate seed, but it may not be consistent enough to get the plants established.  In fact, they may die for lack of further rain or be delayed.

Delayed germination means that soil is cooler and there is less growth. The real growth may occur after there has been sufficient rainfall by January and February when the soils are cooler and there is even less chance for growth

So when rainfall is doing its worst, there's no effective cover.  Or what cover there is, is what has germinated from “native” seed.  It may not have the characteristics you want for management: low stature, low entanglement with the trees, low water use, holds the soil without holding up harvest, etc.

So what do you do?  There are several approaches. You can move the sprinklers out into the middles and irrigate up the seed.  If you are in a limited water situation, you can do alternate middles, not cover cropping the whole area, or every third middle.  Whatever it takes to break the surface flow of water.  Or you can turn to mulching.  Put down sufficient mulch in a middle or every other middle to break overland water flow. 

Cover cropping is easier than mulching, but it takes water and timing.

Below are two websites with descriptions of cover crops and how to distinguish them from “weeds”.  Often a good cover can be the residential weeds. A low–lying cover allows pickers in to get lemons without mess and fear of snakes.  It also means that it can be more easily treated (mowed, weed whipped) at the end of the rainy season to reduce fire hazard.

1)      Characteristics of different cover crops

http://asi.ucdavis.edu/programs/sarep/research-initiatives/are/nutrient-mgmt/cover-crops-database1

2)      Weed identification from the UC IPM

http://www.ipm.ucdavis.edu/PMG/WEEDS/broad_preview.html

APHIS is proposing to amend its fruits and vegetables regulations to allow the importation of avocados from continental Spain into the United States. (This proposal does not include avocados from the Balearic Islands or the Canary Islands.)

As a condition of entry, avocados from Spain would have to be produced in accordance with a systems approach that would include requirements for importation in commercial consignments; registration and monitoring of places of production and packinghouses; grove sanitation; and inspection for quarantine pests by the national plant protection organization of Spain.

Consignments of avocados other than the Hass variety would also have to be treated for the Mediterranean fruit fly (Medfly) either prior to moving to the United States or upon arrival prior to release. Consignments would also be required to be accompanied by a phytosanitary certificate with an additional declaration stating that the avocados were grown and inspected and found to be free of pests in accordance with the proposed requirements.

This action would allow for the importation of avocados from Spain into the United States while continuing to provide protection against the introduction of quarantine pests.

We will consider all comments that we receive on or before April 1, 2013.

You may submit comments by either of the following methods:

• Federal eRulemaking Portal: Go to http://www.regulations.gov/#!documentDetail;D=APHIS-2012-0002-0001
  
• Postal Mail/Commercial Delivery: Send your comment to
  Docket No. APHIS-2012-0002
  Regulatory Analysis and Development
  APHIS PPD Station 3A-03.8
  4700 River Road Unit 118
  Riverdale, MD 20737-1238

Supporting documents and any comments we receive on this docket may be viewed at http://www.regulations.gov/#!documentDetail;D=APHIS-2012-0002-0001 or in our reading room, which is located in room 1141 of the USDA South Building, 14th Street and Independence Avenue SW., Washington, DC. Normal reading room hours are 8 a.m. to 4:30 p.m., Monday through Friday, except holidays. To be sure someone is there to help you, please call (202) 799-7039 before coming.

Sonia Rios, Area Subtropical Horticulture Advisor, Riverside & San Diego Counties

Henry Herrera, Forester, California Department of Forestry and Fire Protection (CAL FIRE)

Gary Bender, Subtropical Horticulture Advisor, San Diego County

This is a reposting of a blog from 2015 that is still pertinent today.

Vegetation Weed Management

Vegetation weed management is something CAL FIRE feels strongly about, especially during times of drought.  CAL FIRE interprets this type of weed management as a fuel reduction or fuel modification practice which helps growers create defensible space around their habitable structures or groves. Defensible space is a buffer between property (i.e. homes, groves, infrastructure) and plants (i.e. brush, trees, etc.) or other items surrounding the property to be protected that could catch fire and act as fuel.

Defensible space is needed to slow the spread of a wildfire and improves the safety of firefighters defending properties. Defensible space reduces the fire risk and creates space that firefighters can use as a tool to work safely and suppress a wildfire and to defend a structure or other property in the event of a wildfire.  Fuel modification/reduction projects include defensible space around structures, fuelbreaks along roads and groves, ridge tops, or around property lines and other fire fuel reduction activities that lessen the risk of wildfires to communities.  Fuel reduction projects usually remove cut vegetation from the site either through burning or haul-away methods.  Fuel modification projects usually leave the cut vegetation on site in the form of wood chips or scattered material.  These types of projects break up the lateral and horizontal vegetative continuity which reduced rates of fire spread, fire intensity, and flame heights which increase firefighter's chances of suppressing fires.

Prunings piled in draws within or around the groves can also be particularly damaging because they are dried vegetation which pose an increased fire danger due to the fact that piles of vegetation can easily ignite and smolder for a long time creating a challenge to firefighters.  Vegetation that is piled in draws or other waterways can also obstruct water flow during rain events which could lead to erosion and flooding.

How to Keep our Growers and Groves Safe

Preventative Measures:

These preventive measures can be suggested for groves in areas of high fire hazard are often counter to other measures which might reduce erosion or improve root disease control, so a balance should be strived for (Goodall 1965):

1. Remove all combustible material from around the trunks of the trees for a distance of two to three feet.

2. Prune off low-lying limbs, those that are low enough to accumulate more than the normal inch or so of leaf mulch.

3. Remove from the orchard all broken limbs, deadwood and other combustible debris.

4. Clear brush, trees and other heavy vegetation away from edge of orchard for a distance of at least 50 feet.

5. Do not pile brush or other combustible material in draws or canyons, or around the edges of groves.

6. Apply sprinkler water for as long a period in advance of the fire as possible so as to have everything wet.  Water during the fire would obviously be desirable but often is lacking because of lack of pressure or speed of the movement of the fire.

7. Use steel pipe and risers for above ground sprinkler systems.

Growers can also look for other tips on preventive measures on the Cal Fire website: http://calfire.ca.gov/fire_prevention/fire_prevention.php.

After a Fire

It is nearly impossible to determine the extent of damage to the wood of the tree immediately after the fire.  The most critical part of the tree is the main trunk.  The bark from just above the ground to about two feet high can become hot enough to kill the cambium, the growth layer between the bark and wood.  When the cambium is killed, the tree is girdled and will die (Goodall 1965). Suckers will eventually grow from below the graft union of these trees; these suckers can then be grafted to an appropriate variety.  However, many trees in the grove may still be alive internally because there was a quick burn through the grove, or the leaves merely died from heat generated by burning surrounding trees, vegetation and mulch (Bender 2012).

According to Bender (2012), fire damaged trees that have leaves that are completely brown do not use water, but there may be trees that have escaped the fire in the irrigation block and these need to be irrigated as soon as possible.  The first thing to do is to repair the irrigation system.  Risers, sprinklers, pressure regulators and poly-hose on the ground are probably melted and should be replaced as soon as possible.  PVC pipe that is buried underground is usually fine, depending on its depth and fire intensity.  If part of the trees in the irrigation block are alive and partly damaged, it would probably be best to roll out poly-hose onto the damaged trees and set up a separate irrigation block.  As the trees recover and start using water, you can eventually go back to the permanent system.

Trees that are heavily damaged do not use as much water (therefore transpiration is reduced significantly), but their feeder roots need to be watered and short irrigations should occur to replace water evaporating off of the soil surface.  Start with a one hour irrigation and monitor the soil closely, irrigating too heavily could lead to the onset of root rot.  Do not irrigate damaged trees on its original schedule; a ten hour irrigation will not “force” the trees to grow back sooner, and roots could be easily damaged by lack of oxygen in the lower soil profiles due to water accumulating and the soil becoming saturated.

Every fire is different, so experiences vary. It may take several months to be able to observe whether the bark is killed all the way to the cambium or not.  Thus, we recommend against pruning until new growth appears to indicate where the wood is alive.  According to Bender (2012), an alternative method which is rather unique to the avocado industry, the burned trees can all be stumped immediately and allowed to re-grow.  “Stumping” is a normal practice in the industry when avocado trees have reached such heights that fruit is high off the ground and picking becomes difficult.  In many of the groves that were burned, stumping was probably needed anyway; therefore after a burn stumping would be a reasonable alternative for many growers.

Fire damaged trees should be whitewashed on the south side of the limbs.  Goodall (1965) also suggests to save expenses, fertilizer and pest control can be postponed for a year or so.

According to Bender (2012), if the goal of the grower is to bring trees back into production as soon as possible, the avocado tree will usually recover production faster if the grower is patient and prunes only the dead wood three months after the fire.  Unfortunately, this practice creates a permanent problem in the grove as far as irrigation scheduling and application.  Mixing full-grown trees (untouched by the fire) with partially pruned trees (and stumped trees) in an irrigation block means that some trees will be over-watered, or under-watered, depending on their size.  Adjustments can be made in the sprinkler sizes, but generally this is an undesirable cultural practice due to every tree needing a different rate of irrigation and can this can also add to labor and equipment costs.

If the goal is to reduce the size of all trees in the irrigation block to a manageable size, then stumping the block immediately after the fire is the best solution.  Trees will be out of production for two years and have about 50% production in the third year, and some re-grafting may have to be done, but fertilizing and watering properly is manageable.

A third option could be to scaffold all trees in an irrigation block to 12' in height.  This would get rid of a lot of dead wood immediately, and might allow the trees to come back in production faster than the stumping the trees.

It is helpful for growers to know the value, for insurance purposes, of trees that are lost due to fire. Determining the value of a tree in an orchard is not a simple matter because one must take into account the income lost if the tree had been producing, as well as the costs of planting and maintaining the new tree. Also, the income from the new tree, once it comes into bearing, helps to defray the costs involved in bringing the tree to maturity. Examples can be seen here at the UCCE website: http://ceventura.ucanr.edu/Agricultural_Threats/Fire_Information/.  Assessing tree loss can be calculated at the UC DAvis Ag Econoimics website: https://coststudies.ucdavis.edu/en/tree-vine-loss/

Record-low rainfall, extreme heat and a statewide drought has caused a significant increase in wildfires and a need for Californians to participate in fire-prevention tactics, including growers.  According to CAL FIRE, in order to prevent wildfires and property damage, homeowners are asked to maintain “defensible space” around their house and groves year round and not use powered equipment outdoors when it's hot, dry or windy. Wildfire prevention such as defensible space, keeping properties cleared of dried vegetation, and public education is the best long term solution.

Photo 1: An inmate firefighter crew battles a fire in an avocado grove outside Fallbrook, California, May 14, 2014. Photo: Sandy Huffaker

Photo 2: Low intensity fire damage on avocados. Photo: U.S. Today

Photo 3: A CAL FIRE inmate crew fighting a wildfire in an avocado grove, May 2014. Photo: Sandy Huffaker

Work Cited

Bender, Gary. 2012. Recovery from Fire Damage in Avocado Groves. UCCE San Diego Website: cesandiego.ucanr.edu/files/54279.doc. [Accessed June 16, 2015].

Goodall, G. E. Avocados and the Coyote Fire. California Avocado Society 1965 Yearbook 49: 81-83.

Ventura Co UCCE publication. Calculate Cost of Fire Damage to Avocado and Citrus Trees. Growing Avocados in Ventura County Reference Book. UCCE Ventura

Sugary exudate on trunks or branches may indicate a PSHB attcak.

Through DNA sequencing, UCR entomologist have determined that we have two separate invasions of the Polyphagous Shot Hole Borer (PSHB) here in southern California attacking avocados. One invasion in San Diego County and the second in Los Angeles County. UCR entomologist, Richard Stouthamer states the two are different species genetically, but morphologically they are indistinguishable, luckily they both respond to the same lure. They do however carry different fungal symbionts with them.

The species that has invaded San Diego, is now known as the Kuroshio shot hole borer, and is originally found only in two places in the presumed native range: highlands of Taiwan and in Okinawa. The species of beetle that has invaded Los Angeles County will now be known solely as the Polyphagous, its native range is in Vietnam, South China, North Thailand, low lands of Taiwan and Okinawa, this species has also invaded Israel and South Africa.

These Shot Hole Bores are Ambrosia beetles, a specialized group belonging to the family Scolytidae. The shot hole borer was first found in Whittier Narrows in Los Angeles County in 2003. From 2003-2010 the beetle was found on a few ornamental trees, then in 2010 it was the presumptive cause of the death of a large number of box elder street trees in Long Beach. In 2012 the beetle was collected from a backyard avocado tree in South Gate, and from several tree species at local botanical gardens, since then it has been causing havoc in the avocado industry. It now appears to be established in Los Angeles, Orange, San Bernardino and Riverside Counties, and is expanding its range in San Diego County. A single beetle was caught in Santa Cruz County in 2014 and has now been found less than 2 miles from Ventura County line.

 The beetle is dark brown to black and tiny, with females between 0.07 and 0.1 inches long, and males even smaller, usually about 0.05 inches long. They differ from the bark beetles in this family in several ways. While bark beetles burrow in the phloem layer or at the juncture of the bark and sapwood, ambrosia beetles bore through the bark and into the sapwood. The shot hole bores are highly specialized and feed on fungi that they cultivate on the walls of the tunnels. In the case of the shot hole bores, they transport the pathogenic fungus, Fusarium euwallacea and both the adults and larvae feed on the fungus.

Females are typically larger, 1.8-2.5 mm long

Shot hole borers attacks hundreds of tree species, but it can only successfully lay its eggs and/or grow the fungi in certain hosts. These include: Avocado, Box elder, California sycamore, Coast live oak, White alder, Japanese maple, and Red willow. Visit http://eskalenlab.ucr.edu/ for the full list. Infection with the fungus can cause a dry or wet and oily dark stain surrounding the entry holes, discolored wood, leaf discoloration and wilting, and dieback of entire branches. In box elders and avocados, a while crusty ring of sugar, also called a “sugar volcano” can be produced. Frass (wood dust left behind from boring) may be produced, but because this can quickly dissolve if it rains it can be missed sometimes. If the barked is scraped away, dark dead tissue may be found around the galleries.

Currently there is no cure for the fungus once the tree has become infected. Chipping and solarizing/tarping infested wood can help to limit the spread of the beetle/fungus. Wood should be chipped to pieces smaller than 1". Don't forget to sterilize pruning tools between uses to avoid spreading the fungus.

If you suspect you may have the shot hole bore in your grove please feel free to contact your local CE Farm Advisor/Specialist.