Ventura County UCCE Farm Advisor, Ben Faber gives advice on how to identify and treat root rot in avocados.

How to identify root rot and treat it?

The canopy is thinning.  The leaves are small and yellow.  There is dieback in the canopy, with leafless tips on the branches.  You dig around under the canopy in the wetted area of the sprinkler and you can’t find roots within 6 inches of the soil surface or if you do find them, they are black.  There is little mulch under the tree.  There are weeds growing under the tree.  All these are signs of root rot disease.  But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water.  At this point, gardeners may unfortunately water the tree more, which only makes conditions worse for a diseased tree; adding more water to a tree that can not easily take it up, creates asphyxiation conditions.

So what do you do if you have disease?  First of all, make sure you are irrigating to the needs of the tree by checking soil moisture before irrigating and ensuring the tree is not receiving supplemental water from another area such as a lawn sprinkler.  Add and maintain a good thick layer of woody mulch under the canopy.  Adding gypsum (15-20 pounds per tree), evenly spread under the canopy, can also help.  There are fungicides available from retail nurseries, but reviewing and modifying irrigation practices and maintaining a mulch layer are the two most important things you can do.

Read on for details on mulch and irrigation.

Mulch and avocados?

“I just raked up all the leaves under the avocado and it looks so nice,” you say.  PUT THEM RIGHT BACK.  The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots.  In fact, the roots will actually colonize the rotted leaves as if it were soil.  This mulch is also a first line of defense against root rot.  The decomposing leaves create a hostile environment to the microorganism that causes the disease.  The mulch also helps to reduce evaporative loss of water and therefore reduces water needs.  Commercial growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away.  The key to remember is that the mulch should be kept at least 6 inches away from the trunk to avoid collar rot, which can be caused by keeping a moist mulch against the trunk.

How to irrigate avocados

Mature avocado trees may be large above ground, but they have very shallow roots mostly in the top 8 inches of soil.  The tree therefore does not have access to a large volume of stored water.  As opposed to a deep-rooted walnut, avocados require frequent, small amounts of water.  A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons.  An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall.  Proper irrigation is the best way to keep the avocado from getting root rot.  Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common.  And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well.  Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.

Dr. Sabrina Drill, our cross-county (LA & Ventura) Natural Resources Advisor has developed guidelines for creating and maintaining fire-safe, environmentally-friendly landscapes in the wildland-urban interface that minimize the use and spread of invasive plants.

The information available on the UCCE Los Angeles County website has the potential to reduce the chances your home will be damaged or destroyed by wildfire. By clicking on the “CLICK HERE to get a copy of the 2009 Ventura County SAFE Landscapes Calendar and Guidebook.” You will find great tips and practical measures to implement around your home that are parsed into small tasks to tackle on a monthly basis. Further links for additional information are available at the bottom of the page.

The Ventura County UCCE office has some copies left of the 2009 fire safe landscaping calendars if you would like a hard copy version. While the year is almost over, the calendar was designed to remove the part that will soon be outdated leaving the great suggestions and tips behind for future reference. Please contact our office if you would like a copy.

As with many things in life, chances for long-term success with citrus are increased with knowledge and planning. Even the size of the plant put in the ground has an impact on long-term growth and return on investment. Ventura County UCCE Farm Advisor, Ben Faber shares his knowledge with us.

What size plant should I buy?

The longer the plant has been in the container, the longer it takes for the plant to adjust to the soil after it has been planted.  The smaller the plant, the more rapid the growth once it is put in the ground and survives -- a 5 gallon container- grown lemon will have outgrown the 15 gallon container in 3 years.  This has been shown consistently with all manner of container grown plants……..and they are cheaper.

One vital consideration is the type of variety to plant. Please remember to use only rootstocks that are California-grown certified trees that are known to be free of disease. It is illegal to bring in citrus trees or cuttings into California from other states or countries because they may be infected with disease or insects.

Rootstocks

There are many different rootstocks available to growers.  Choose rootstock based on characteristics that are important to the growing site, such as greater nematode resistance, salt resistance, disease resistance, etc.  The retail nursery typically sells whatever rootstock the wholesale nursery propagates. Wholesale nurseries do not all use the same rootstocks, but use those that they feel grow best for them.  In some cases, a retail nursery may be able to special order a rootstock for a special situation.  You can always ask.

There is one choice that the buyer can make, though – whether the rootstock is dwarfing or not.  The ‘Flying Dragon’ rootstock creates small tree, under 6 feet and it is very slow growing.  It especially lends itself to container culture.

For home growers, another important consideration is where to plant the trees. Irrigation needs are an important piece of this puzzle. Keep the following in mind while looking for a planting location:

Growing citrus in a lawn

Trees don’t belong in lawns.  In California, we irrigate.  Do you irrigate to the needs of the lawn or to the tree?  Frequently, lawns are irrigated by timers, putting a short burst of water on.  Trees like a deep watering.  Shallow watering leads to an accumulation of salts in the tree’s root zone and salt burn results.  If possible, keep a 6-foot turf-free area around the trunk.  And best of all, irrigate the tree separately from the turf and ensure the lawn sprinklers do not wet the trunk, which can lead to crown rot in the tree.

Citrus can be grown in containers. Ben offers the following suggestions for the best results:

Container grown citrus

Citrus grows well in containers, especially if you choose varieties like ‘Meyer’ lemon which is a less aggressive tree or use ‘Flying Dragon’ dwarfing rootstock on one of the other citrus varieties.  There is a long history of orangeries in Europe, where full sized trees were grown outside in containers in the warm weather and then moved into large greenhouses when it got cold. Half barrels and terra cotta pots can be used, but if a large container is used and you want to be able move it, put the container on some wheels first.  Fill the container with a good quality potting mix and plant your tree.  Containers dry out much faster the soil grown trees, so stay on top of the irrigation.  When irrigating, make sure water comes out of the bottom of the pot to avoid salt accumulation in the root zone.  Prune as necessary to keep the canopy in balance with the pot or pot up to the next size.

Additional information can be found on the Ventura County UCCE website.

Healthy citrus

Ventura County UCCE staff research associate, Maren Mochizuki explains how weather data is collected and shared.

The California State Irrigation Management Information System (CIMIS) is a program of the California State Department of Water Resources Office of Water Use Efficiency.

CIMIS consists of a network of more than 120 weather stations throughout California that measure parameters such as air and soil temperature, precipitation or rainfall, wind speed, relative humidity, amount of solar radiation, and much more.  This data is recorded every minute at most stations; data from some stations are available from as far back as 1982!

Growers use this data to aid in irrigation planning and scheduling for their crops but data from any of the stations is available free to the public the day after the data has been recorded.

To find stations nearest to you and to view a sample report with data from the last seven days, please visit: http://wwwcimis.water.ca.gov/cimis/frontSampDailyReport.do

Photos of CIMIS station