Citrus: when things go wrong
Making lemonade from frozen lemons?
written for the Davis Enterprise, December 31, 2009
So many questions about citrus lately! That's not surprising when we get to 20 degrees. That's right: the official low temperature reading for Davis on the morning of December 8, 2009 was 20 degrees F, with a low of 23 degrees on December 9.
Your readings may have been a bit higher: that weather station is about a half-mile west of Highway 113, in an open field on the UC Davis campus. Gardeners in town reported temperatures in the low 20's. Whenever we get below the mid-20's we can expect some damage to citrus leaves and fruit. And nursery professionals, master gardeners, and landscapers can all expect the public to be concerned and have lots of questions.
Fortunately the freeze only lasted a couple of nights. Damage from freezing weather is a function of how long the cold lasts as well as the absolute temperature. In the memorable freeze of 1990, temperatures dropped below 20, and nights were below freezing for nearly two weeks!
What happens to citrus when we get freezing weather?
Young trees can be severely damaged. Tender new growth on green, non-woody branches is most vulnerable to the desiccating damage caused by freezing. Plants less than a year old have very little of the tougher wood, and may be so badly harmed that you might as well replace them.
A severely damaged young lime tree. I forgot to protect this tree, which had been planted just last September! In a last-ditch effort to save the stem and roots, leaves were raked over and around the plant (in the middle of the night). This lime is on its own roots, so we will see in the spring if it resprouts from the roots. A grafted tree would probably have to be replaced, as the new growth would likely be rootstock.
But wait until spring before you cut or pull them: new growth might emerge from the lower branches and trunk. Just make sure it is arising from above the graft union (visible on most citrus at about 6 - 12 inches above the ground). Anything from below that is rootstock and will not have edible fruit.
Note: any citrus grown on their own roots might resprout from the ground and be fine. Meyer lemon is most commonly grown on its own roots. Most other citrus are grafted onto rootstock.
Older trees often put on a lot of tender new growth in the late summer and fall, and the freeze may have withered it. This is pretty harmless, the equivalent of a light pruning. But don't remove any of that damaged growth, as the cut ends you leave would be vulnerable to further freeze damage in the event of another episode. That applies to most subtropical plants: the "burnt" foliage helps to protect the remaining leaves and stems below. So just put up with the unsightly appearance until frost danger is past in March.
Damage to fruit is less obvious.
Discolored areas appear on the rind: look for irregular, light-colored spots. If we get rain after the freeze (as we did) there may be rot already developing at these spots. Cut an orange in half and you may see that the segments have pulled away and separated slightly. There might even be some crystalline material present. Ultimately the fruit either dries out or rots. Fruit on the outer part of the tree is less protected by foliage, so it is more vulnerable to freeze damage.
Frost-damaged leaves on a navel orange. Citrus leaves curl when stressed, either by cold or heat. Cold causes desiccation (drying) of tissue, causing the leaves to look "burnt."
Citrus growers have some sophisticated monitoring and sampling techniques for assessing the damage to their crops. Researchers have found that black lights highlight those spots quite readily - the damaged areas glow like those posters from the 1970's -- allowing them to assess the damage in the middle of the night and make a quick decision about harvesting and trying to salvage the crop. Get that black light out of the attic, dad! The good news is that this short freeze did little damage to California's navel orange crop. A variety of frost protection measures allowed growers to boost local temperatures 3 - 4 degrees and minimize harm to fruit and foliage.
Possible freeze damage on a navel orange. Look for yellow spots, which then develop into pitted areas and often rot if warm(ish) wet weather follows.
Citrus varieties vary in hardiness.
Kumquats are very tough trees, tolerating temperatures even in the teens with little damage. Mandarins are hardy enough to be grown as a commercial crop over in the foothills near Loomis.
On the other end of the spectrum, Mexican limes (aka Key limes) are so tender they will be harmed at 30 degrees F, and aren't really recommended. Bearss is a lime variety that is somewhat hardier, but is still among the more tender types.
Meyer lemons have fruit with thin skin that is easily damaged, even though the plant itself may show little cold damage. You may wish to pick the fruit borne on the outer part of the tree when very cold weather threatens.
Commercial true lemons such as Eureka and Lisbon are prone to more foliage damage, but the fruit itself has thicker, tougher skin.
Oranges, grapefruits, and tangelos are intermediate, generally tolerating our winters well and requiring protection only during protracted freezes.
Drainage problems, fungus, and bacteria
With all this cold, people have forgotten about the other major weather event of 2009: the big rainstorm that dumped more than three- and a-half inches of rain in the area on October 13. Early fall rain causes problems with citrus, and a few examples have come my way in recent days. Neither is very common, but I expect we'll see more than usual this year.
Citrus brown rot (a Phytophthora species) is a fungus that rots the fruit right on the tree, just as it is ripening. (This is not the same as the brown rot fungus that attacks apricots and other stone fruits; that is Monilinia species). The fruit rots rapidly on the bottom and develops an unpleasant odor. Infection quickly spreads to nearby fruit in storage.
Citrus brown rot is worse when we have early fall rain, on older trees, and on varieties whose fruit hangs on through the year such as lemons. Copper sprays applied before the rainy season (i.e., October) will help keep the fungus from splashing up onto the fruit. At this point in the season, all you can do is pick up and discard rotting fruit (yuck), and prune your trees up a couple of feet to keep foliage clear from the ground.
Citrus blast is a bacterial disease (Pseudomonas syringae) that infects during wet weather, causing a rapid dieback. Leaves and twigs wither and die. Prune out the affected portion in the spring after wet weather ends. Again, copper sprays in the fall before the rainy season can help prevent infection.
Finally, that much rainfall all at once is hard for our soil to absorb, particularly in the parts of town with higher clay content (Stonegate, north Davis, Binning Tract, North Davis Meadows). Saturated conditions while the soil is still warm can cause infection of roots by fungus. This can lead to various symptoms, most commonly yellowing of the leaves and apparent nutrient deficiencies.
Navel orange leaves curled from freezing weather, yellow due to nutrient deficiencies -- possibly caused by poor drainage, overwatering.
What is really happening is the damaged roots are unable to take up minerals. Some are needed by the plant in very small quantities (micronutrients), but are important to the green leaf color.
Overall yellowing of the tree can be caused by watering too often during summer, lack of fertilizer, root damage from poor drainage, cold weather -- or all of the above! Fertilizing during winter isn't likely to be very effective, as the soil is too cold for the plant to make much use of it. But a tree with yellow foliage should be put on a regular fertilizing schedule as soon as weather warms in spring, and watered deeply and infrequently in summer.
Huanglongbing (HLB) and the Asian citrus psyllid
I would be remiss if I didn't update you on the major pest/disease problem threatening California's citrus industry and home gardeners. Citrus greening disease has not been found in California yet, but the insect that spreads it has now been found in six counties in Southern California, where quarantines are now in effect.
When the Asian citrus psyllid became established in Florida, the disease quickly followed - probably initially imported on infected trees. Florida growers are losing more of their trees each year, and organic citrus production is nearly impossible where the insect and disease are present. So please DO NOT bring citrus leaves, fruit, or trees from Southern California or any other state.
The trees you buy from licensed nurseries are disease-free. The UC Riverside Citrus Program is maintaining propagation stock of disease-free trees and providing that budwood to growers statewide. Do not buy or accept a tree from any non-licensed source. This disease ruins the fruit and slowly kills the tree, and cannot be controlled in infected trees. They simply have to be pulled and burned.
Click here for pictures and more information: saveourcitrus.org
In spite of these occasional problems, citrus trees are pretty carefree. Choose suitable varieties for the Sacramento Valley. Plant in a sunny location with reasonable drainage, water deeply and infrequently, and fertilize seasonally or monthly when the trees are young. Take some measures to protect them from freezing weather, and fertilize older trees if they show signs of deficiencies. Prune only if you want to control the size of the tree. Ornamental, edible, and emblematic of California!
© 2009 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
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