Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: weather

Ready! Set! Split! Navel Orange Time.

These are hard days for navel oranges.  Drought stress.  Salt stress due to drought. Then a heat wave in July that messed the trees up.  And now we head into a weird fall with maybe rain. Maybe no rain. Maybe a little rain. This is ripe for navel splitting. This time of year when they are starting to build sugar, they are also ripe for splitting.

Years of drought, and a stressed tree are a perfect set up for navel oranges and fruit splitting.
The days have turned cooler and suddenly out of nowhere there is rain. That wonderful stuff comes down and all seems right with the world, but then you notice the navel fruit are splitting. Rats! No, a dehydrated fruit that has taken on more water than its skin can take in and the fruit splits. This is called an abiotic disease.  Not really a disease but a problem brought on by environmental conditions.

Fruit splitting is a long-standing problem in most areas where navel oranges are grown. In some years, the number of split fruit is high; in other years it is low. Splitting in navel oranges usually occurs on green fruit between September and November. In some years, splitting may also occur in Valencia oranges but it is less of a problem than in navel oranges.

Several factors contribute to fruit splitting. Studies indicate that changes in weather including temperature, relative humidity and wind may have more effect on fruit splitting than anything else. The amount of water in a citrus tree changes due to weather conditions and this causes the fruit to shrink and swell as water is lost or gained. If the water content changes too much or too rapidly the rind may split. In navel oranges the split usually occurs near the navel, which is a weak point in the rind.

Proper irrigation and other cultural practices can help reduce fruit splitting. Maintaining adequate but not excessive soil moisture is very important. A large area of soil around a tree should be watered since roots normally grow somewhat beyond the edge of the canopy. Wet the soil to a depth of at least 2 feet then allow it to become somewhat dry in the top few inches before irrigating again. Applying a layer of coarse organic mulch under a tree beginning at least a foot from the trunk can help conserve soil moisture and encourage feeder roots to grow closer to the surface.  

If trees are fertilized, apply the correct amount of plant food and water thoroughly after it is applied.  If the soil is dry, first irrigate, then apply fertilizer and irrigate again.

navel split
navel split

Posted on Friday, November 2, 2018 at 4:55 AM
Tags: citrus (310), irrigation (76), navel orange (2), rain (17), water (48), weather (8)

Tree Crop Disaster Insurance

Fire, Flood, Freeze, Fytophthora. It seems like there is disaster around every corner these days. This can be tough for tree growers with a lot of investment in the ground. There has been a special USDA crop insurance available for ‘Hass' avocado growers for several years. For a reasonable amount, a considerable amount of disaster insurance can be obtained:

This program is not available for non ‘Hass' avocado, although that may change in the future. It is also not available for many other perennial crops like pomegranates and persimmons and may only be available for some crops like blueberry and citrus in some counties and not in others.

However,there is disaster insurance available for many fruit crops in all counties in California. This program is The Noninsured Crop Disaster Assistance Program (NAP), administered by the U.S. Department of Agriculture (USDA) Farm Service Agency (FSA). It provides financial assistance to producers of noninsurable crops against natural disasters that result in lower yields or crop losses.

The 2014 Farm Bill specifies that an individual's average adjusted gross income (AGI) cannot exceed $900,000 to be eligible for NAP payments.

Noninsured Crop Disaster Assistance Program

Tree Assistance Program

2018 NAP Application Closing Dates


Unfortunately, the closing dates for crops like pomegranates, persimmons, and blueberries closed October 1, 2017 for this year, but avocados and citrus are still open until February 2018.

For more information about the program, contact:

*ù...Daisy Bandaù*.

Program Technician

Santa Barbara-Ventura County | Farm Service Agency

920 East Stowell RD | Santa Maria, CA 93454

(: (805)928-9269 Ex. 2 |Fax: (844)206-7008

Insurance Survey

The National Center for Appropriate Technology (NCAT) is seeking input from growers across the nation concerning crop insurance. NCAT is hosting an online survey, which takes 20 – 30 minutes to complete, that will compile grower and rancher opinions, attitudes and concerns regarding crop insurance. The results will remain anonymous and will be used to shape crop insurance recommendations that NCAT, in partnership with a number of other agricultural organizations, will present to the United States Department of Agriculture concerning organic, diversified and specialty crop producers.

NCAT will pay those who complete the survey $20.

Growers interested in participating in the anonymous crop insurance survey can do so by clicking on this link.

IMG 2031
IMG 2031

Posted on Friday, November 3, 2017 at 6:39 AM
Tags: disaster (4), fire (29), flood (2), help (1), insurance (3), weather (8)

Rain ???? Weather Forecasting Failure

Last Sunday was forecast as 20% chance of rain in Ventura, but it didn't happen. Today, Friday it was supposed to rain, but it's barely a drizzle.  My car's dust has just been redistributed. Could a better prediction be forecast?  According to Cliff Mass it certainly could.

According to Mass, other countries like the UK and the European Union have invested heavily in computing power that allows greater prediction of local weather. In fact, the EU can forecast weather in the US better than National Weather Service.  The problem at the Service is not for lack of imagination and smarts but a lack of funding for super computers that can do the improved predictions.


Posted on Friday, October 28, 2016 at 12:52 PM
Tags: forecasting (3), rain (17), weather (8)

Avocados in June

No matter where they grow in California, June is a month when avocados are being watered on a regular schedule. How regular that schedule is should be carefully reviewed by the irrigator.  In 1991-'92, right along the coast in a Ventura irrigation plot, we applied 32" of water, but in '92-'93 we put on only 26".  Same trees, nearly the same size, but a 23% difference in applied amounts dictated by differences in water demand due to different weather.  The irrigation schedule we use is driven by tensiometers and a CIMIS weather station.  The station generates reference evapotranspiration values which tell us how much water to apply at an irrigation, and the tensiometers are used to verify whether the trees are doing well by the schedule.  Irrigation on a fixed schedule, such as once a week for 24 hours, is going to guarantee that on average you will be either under or over irrigating at each irrigation.  Using some soil-based measure, such as a soil probe or tensiometer can assure an irrigator that trees are getting the appropriate amount of water when they need it.  If you haven't done so, the irrigation calculator available at the website can be quite useful in guiding an irrigation schedule - - check it out.  You also need to correct for salinity accumulation.

In orchards which have not closed canopy yet, weeding is an ongoing activity. In a research plot, we are using tensiometers to monitor the effects of weeds, bare soil and chipped yard waste mulch around trees.   In weedy plots soil moisture profile rapidly show 30-40 cbars of tension at 6", whereas the bare and mulched plots can go much longer before showing 40 cbars.   Centibars is a measure of moisture tension, the higher the value, the drier the soil.  As trees get older they make their own leaf mulch and shade which limit weed growth.  There is no question that a cover crop can improve soil conditions through reduced erosion, improved water infiltration, and possible reduced disease and pest problems.   These soil improvements tend to improve tree growth and orchard productivity. But, if water is the primary issue, weeds and a cover crop can add considerably to water use in an orchard, especially a young one.  Weed control through the use of mulches and herbicides can effectively reduce the water requirements of trees.

June is still a good time to replant an orchard. The soils are warm enough to give the trees a good start and there is enough fine weather left for them to establish before winter comes.  Late plantings (September, October) are discouraged because the root-shocked plant sits in a cold, wet soil through the winter and becomes a prime candidate for root diseases.  Especially in a replant situation, it is a good idea to start them off with a fungicide with one of the phosphonate products, to give them some protection until they get established.  The best time for to apply the material to do its job on older trees is when there is a good root flush of growth which occurs after the leaf flush in spring and fall.

When replanting, try as much as possible to avoid interplanting between older trees. The different water requirements of the young and old trees is such that one or both will be stressed because they need different schedules - less but more frequent for the young trees.  Attempts can be made to put together a system where the older trees remain on the 10 or 15 gpm mircosprinkler and the young trees are put on a 1 gpm dripper.  This still cannot be an ideal situation, since the needs for application frequency are still different between the small and big trees.  The best thing to do is to clear out trees within an irrigation block and replant, or replumb a block with a new valve so that small new block can be irrigated differently from the older trees.  Where clonal rootstocks fail in a root rot replant situation, it is invariably where water control is lacking or poor.

As we all know, this has been a long dry spell in the avocado growing areas along the coast. With the levels of salt in our waters, it's important to have some kind of a leaching program to ensure that salts do not accumulate in the root zone. Each winter, rain leaches the accumulated salts from the previous irrigation season and starts the orchard off to a good start. These years it hasn't happened and one of the things that can affect the trees is a stress.  This is a salt stress that is most pronounced at the end of irrigation lines and where low pressure results in low output, often at the top of the hill.  Any irrigation system that has poor distribution uniformity is going to have areas where less water than average is applied. 

One of the responses of the trees to salt stress is to exhibit cankers in the branches. These can be silver dollar-sized cankers running in a line up the branch or as diffuse white spots in the branches.  The first symptom is related to bacterial canker and the second is to black streak.  These are not killer diseases, but they are reflective of the tree being under stress.  As soon as the irrigation schedule is corrected, these symptoms can clear up in several months.  If the schedule is not corrected the tress will begin losing leaves and sunburn can result.  The symptoms of these two problems can be viewed at the UC Integrated Pest Management website -  and 

Again these are primarily stress-related diseases and the way to correct the situation is to improve irrigation distribution uniformity and the irrigation schedule. If it goes on too long it can cause problems especially in young trees. When you boil down farming to the basics, the most important activity in the orchard is ensuring proper irrigation.



avocado irrigation
avocado irrigation

Posted on Wednesday, June 1, 2016 at 7:13 AM

Surface inversions cause spray drift

On typical days, the air near the ground is warmer than the air above it. This is because the atmosphere is heated from below as solar radiation warms the Earth's surface. A surface inversion occurs when the atmosphere at the earth's surface is colder than the layer above it. There are four common causes of surface inversions, some of which can occur at the same time. Remember - inversions flow like water:

Advection of cool air:

A slab of cool air slides into and under a warmer air mass. This "drainage inversion" can occur when there are sea breezes, cold fronts or when cool air drains downhill into warmer air.

Advection of warm air:

Warm air flows over cool surfaces and lower layers cool more rapidly than those above.


Shading from trees as well as from rolling terrain can cause an inversion to set in earlier and stay later.

Radiation cooling:

Around sunset, the ground cools rapidly by radiating heat upwards into space. The air in contact with the ground cools by conduction, causing the lowest layer of air to be cooler than higher layers. Air within this "radiation inversion" tends to remains in place.

Radiation inversions create problems for spray operators because they can cause pesticide spray to:

  • stay concentrated for long periods over the target,
  • move with the cool air for many miles when the breeze picks up,
  • drain down slopes and concentrate in low-lying regions,
  • drift unpredictably as the inversion dissipates during the morning

Radiation inversions happen every day and should always be expected to begin 3-4 hours before sunset, reach their apex just before sunrise and then dissipate no longer than an hour or so after sunrise… unless one or more of the following conditions occur:

  • There is continuous overcast, low and heavy cloud.
  • There is continuous rain.
  • Wind speed remains above 7 m/h for the whole period between sunset and sunrise - although even this isn't always true.

Field air temperatures are often very different from local or regional forecasts, so the most reliable method of detecting inversion conditions is to measure temperatures at, and several meters above, the ground. Spray operators can recognize a surface inversion when:

  • there is a big difference between the daytime and night time temperatures,
  • evening and night time wind speeds are considerably less than during the day,
  • sounds seem to carry further,
  • odors seem more intense,
  • daytime cumulus clouds tend to collapse toward evening,
  • overnight cloud cover is 25% or less,
  • mist, fog, dew and frost occur
  • smoke or dust hangs in the air and/or moves laterally in a sheet.

If you suspect there's an inversion, then don't spray. Often, it's right on the label.


Farm Advisor Mark Battany measuring inversions


Posted on Thursday, March 19, 2015 at 1:49 PM
Tags: drift (3), inversion (2), pesticides (23), spray (6), weather (8)

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