The word is getting out. If you have yellowish leaves, cupped/upright and the fruit is small, it may not be Huanglongbing (HLB), but it sure seems like all of my neighbors think so. It could be just lack of water, and in a drought, that could be the most likely cause. But there are other causes of symptoms that might be associated with HLB. Citrus Stubborn Disease causes these symptoms, but also distorted fruit and shriveled, discolored seeds, and bitter fruit like HLB. Under very hot conditions, leaves on some shoots may have misshaped, blunted yellow tips with mottling similar to the nutritional deficiencies seen in HLB. The leaves can have shortened internodes, so there's a bunchy growth habit like in zinc deficiency. Fruit are small, sparse and have early drop. Again, a lot like HLB. Even more so, though Stubborn can cause stunted, thin canopies. Misshapen fruit, though can be caused by bud mites in lemon, chimeras (spontaneous mutations) and Tristeza (a viral disease). Frost damage can add further to the confusion about symptoms that might be associated with HLB.
Citrus Stubborn Disease is a serious disease that leads to reduced fruit quality and yield. It occurs most commonly in oranges, but does show up in other citrus including lemon and mandarin. It's more commonly seen in older trees that were initially propagated with infected tissue, and growing in hot, dry environments. Unlike HLB, it doesn't lead to the death of the tree, just major loss of income. It's caused by a phytoplasma (a bacteria without a cell wall) and is spread by a leafhopper. There are other hosts like mustard and cabbage that can harbor the organism to be spread to new tissue and especially young trees. Warm winters favor the spread of the infected leafhopper, like we have had this winter.
So how do you distinguish between all the possible causes that look like HLB? HLB can be tested for as well as Tristeza. There are no commercial labs that check for Stubborn. It's basically a process of elimination then to decide to test for HLB or to think that the tree has the disease. So, know your trees and their history. Was there a freeze this winter? Know the effects and symptoms of drought and monitor soils and trees for water stress. Check for bud mite. It also takes time from the point of infected Asian Citrus Psyllid invasion to the time when symptoms of HLB start showing up in the tree. So keep your eyes open, but don't assume the worse at this point.
More on Tristeza:
Misshaped fruit from Stubborn
Discolored seeds from Stubborn
Reduced canopy size from Stubborn
Drought Induced Problems in Our Orchards
Abiotic disorders are plant problems that are non-infective. They are not caused by an organism, but through their damage, they may bring on damage caused by organisms. Think of a tree hit by lightning or a tractor. The damage breaches the protective bark which allows fungi to start working on the damaged area, eventually leading to a decayed trunk. It was the mechanical damage, though that set the process in motion.
Too much or too little water can also predispose a plant to disease. Think of Phytophthora root rot or even asphyxiation that can come from waterlogging or too frequent irrigations.
Salinity Effects from Lack of Water
Lack of water and especially sufficient rainfall can lead to salinity and specific salts like boron, sodium and chloride accumulating in the root zone. This happens from a lack of leaching that removes native soil salts from the root zone or the salts from the previous salt-laden irrigation from the root zone. These salts cause their own kind of damage, but they can also predispose a tree to disorders, disease and invertebrate (insect and mite) damage.
Lack of water and salt accumulation act in a similar fashion. Soil salt acts in competition with roots for water. The more soil salt, the harder a tree needs to pull on water to get what it needs. The first symptom of lack of water or salt accumulation may be an initial dropping of the leaves. If this condition is more persistent, though we start to see what is called “tip burn” or “salt damage”. Southern California is tremendously dependent on rainfall to clean up irrigation salts, and when rain is lacking, irrigation must be relied on to do the leaching
As the lack of leaching advances (lack of rainfall and sufficient irrigation leaching) the canopy thins from leaf drop, exposing fruit to sunburn and fruit shriveling.
Leaf drop and fruit shriveling in avocado.
In the case of sensitive citrus varieties like mandarins, water stress can lead to a pithy core with darker colored seeds, almost as if the fruit had matured too long on the tree.
Total salinity plays an important factor in plant disorder, but also the specific salts. These salts accumulate in the older leaves, and cause characteristic symptoms that are characteristic in most trees. Boron will appear on older leaves, causing an initial terminal yellowing in the leaf that gradually turns to a tip burn.
Often times it is hard to distinguish between chloride, sodium and total salinity damage. It is somewhat a moot point, since the method to control all of them is the same – increased leaching. There is no amendment or fertilizer that can be applied that will correct this problem. The damage symptoms do not go away until the leaf drops and a new one replaces it. By that time hopefully rain and/or a more efficient irrigation program has been put in place.
The Impact of Drought on Nutrient Deficiencies
Salinity and drought stress can also lead to mineral deficiencies. This is either due to the lack of water movement carrying nutrients or to direct completion for nutrients. A common deficiency for drought stressed plants is nitrogen deficiency from lack of water entraining that nutrient into the plant.
This usually starts out in the older tissue and gradually spreads to the younger tissue in more advanced cases.
The salts in the root zone can also lead to competition for uptake of other nutrients like calcium and potassium. Apples and tomatoes are famous for blossom end rot when calcium uptake is low, but we have also seen it in citrus. Low calcium in avocado, and many other fruits, leads to lower shelf life. Sodium and boron accumulation in the root zone can lead to induced calcium deficiencies and increased sodium can also further lead to potassium deficiencies. Leaching can help remove these competitive elements.
Drought Effect on Tree Disease
Drought and salt stress can also lead to disease, but in many cases once the problem has been dealt with the disease symptoms slowly disappear. They are secondary pathogens and unless it is a young tree (under three years of age) or one blighted with a more aggressive disease, the disease condition is not fatal. Often times, in the best of years, on hilly ground these diseases might be seen where water pressure is lowest or there are broken or clogged emitters. The symptoms are many – leaf blights, cankers, dieback, gummosis – but they are all caused by decomposing fungi that are found in the decaying material found in orchards, especially in the naturally occurring avocado mulch or artificially mulched orchards. Many of these fungi are related Botryosphaerias, but we once lumped then all under the fungus Dothiorella. These decay fungi will go to all manner of plant species, from citrus to roses to Brazilian pepper.
Another secondary pathogen that clears up as soon as the stress is relieved is bacterial canker in avocado. These ugly cankers form white crusted circles that ooze sap, but when the tree is healthy again, the cankers dry up with a little bark flap where the canker had been.
Drought Effect on Pests
Water/salt stress also makes trees more susceptible to insect and mite attack. Mites are often predated by predacious mites, and when there are dusty situations, they can't do their jobs efficiently and mites can get out of hand. Mite damage on leaves is often noted in well irrigated orchards along dusty picking rows
Many borers are attracted to water stressed trees and it is possible that the Polyphagous and Kuroshio Shot Hole Borers are more attracted to those trees.
And then we have conditions like Valencia rind stain that also appears in other citrus varieties. We know it will show up in water stressed trees, but we aren't sure what the mechanism that causes this rind breakdown just at color break. Could it be from thrips attracted to the stressed tree or a nutrient imbalance, it's not clear?
Water and salt stress can have all manner of effects on tree growth. It should lead to smaller trees, smaller crops and smaller fruit. The only way to manage this condition is through irrigation management. Using all the tools available, such as CIMIS, soil probes, soil sensors, your eyes, etc. and good quality available water are the way to improve management of the orchard to avoid these problems.
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Tip Burn, notice sun burn bottom right hand fruit
Endoxerosis with dried out core
Blossom end rot
Bot gumming in lemon
Black Streak in Avocado
Citrus red mite
Polyphagous Shot Hole Borer damage on avocado
Valencia Rind Stain
Nutrient availability from organic sources has been considered “slow release” by many growers and advisers. This may be true in environments are colder and especially soils are cooler. Organic nutrients are dependent on microbes to break down materials and release those nutrients, and when soils are cold, microbes can't do their thing. Soils in much of agricultural California tend to be warm and lack the freezing conditions that occur in many soils in the continental US. Imagine how much microbial activity occurs in the Mid-West when soils cool down to 32 deg F at a four inch depth and deeper. The top layers of soil are where organic matter accumulates and where most microbial activity occurs. When soils cool below 50 deg F, nitrogen leaching becomes less common, because less activity is occurring which also coincides with much less plant growth.
Soils in coastal California rarely fall below 50 deg F in the surface layers, so microbial activity is ongoing, all year long. So the question is, how “slow acting” are organic fertilizers? A recent study by Tim Hartz, Richard Smith and Mark Gaskell looked at release rates of injectable organic fertilizer and found that much of the nutrient release occurs within about a week after application depending on the formulation and temperature during the study. The results conform to another study that they did where they evaluated the nitrogen release rates of dry formulations of organic fertilizers – compost, manures, feather meal, etc.
Aside from the issues of the higher costs of these materials and their potential clogging, there is the issue of application timing. In the case of avocados and citrus, adequate levels of nitrogen are needed in the trees going into to fruit set in order to optimize set. And then after fruit set, in order to maintain growth into the fast growth period, again nitrogen needs to be adequate. Using organic fertilizers with a rapid conversion to useable forms of nitrogen, means that application timing should coincide with these critical periods in tree phenology or growth cycle.
Using information on organic nutrient management based on work from cold soil climates needs to be carefully evaluated before applying it to California soils. One of the most common problems in organic production is nitrogen management. Part of the problem is the cost of supplemental nitrogen amendments, but also learning to anticipate when that applied nutrient becomes available to the plant. Developing better estimates for local release rates and patterns will better help manage organic nutrient sources.
Summary: Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates additional N fertilization. The increasing use of drip irrigation has created a demand for liquid organic fertilizers that can be applied with irrigation. The N availability of three liquid organic fertilizers was evaluated in an incubation study and a greenhouse bioassay. Phytamin 801 contained fishery wastes and seabird guano, while Phytamin 421 and Biolyzer were formulated from plant materials. The fertilizers ranged from 26 to 60 g·kg−1 N, 8% to 21% of which was associated with particulate matter large enough to potentially be removed by drip irrigation system filtration. The fertilizers were incubated aerobically in two organically managed soils at constant moisture at 15 and 25 °C, and sampled for mineral N concentration after 1, 2, and 4 weeks. In the greenhouse study, these fertilizers and an inorganic fertilizer (ammonium sulfate) were applied to pots of the two organically managed soils with established fescue (Festuca arundinacea) turf; the N content of clippings was compared with that from unfertilized pots after 2 and 4 weeks of growth. Across soils and incubation temperatures, the N availability from Phytamin 801 ranged from 79% to 93% of the initial N content after 1 week, and 83% to 99% after 4 weeks. The plant-based fertilizers had significantly lower N availability, but after 4 weeks, had 48% to 92% of initial N in mineral form. Soil and incubation temperature had modest but significant effects on fertilizer N availability. Nitrification was rapid, with >90% of mineral N in nitrate form after 1 week of incubation at 25 °C, or 2 weeks at 15 °C. N recovery in fescue clippings 4 weeks after application averaged 60%, 38%, and 36% of initial N content for Phytamin 801, Phytamin 421, and Biolyzer, respectively, equivalent to or better than the N recovery from ammonium sulfate.
A 'Meyer' lemon should be quite happy along the coast, unless it gets planted in new soil that has low copper because of high soil pH or high organic matter. And then you wonder what is wrong.
Mild copper deficiency is usually associated with large, dark green leaves on long soft angular shoots. Young shoots may develop into branches which appear curved or “S-shaped," referred to as “ammoniation” usually resulting from excessive nitrogen fertilization. It's actually thought to be too much nitrogen relative to copper in the plant and can be corrected by foliar sprays. Twigs can develop blister-like pockets of clear gum at nodes. As twigs mature, reddish brown eruptions may occur in the outer portion of the wood. It can be quite shocking. Severely affected twigs commonly die back from the tip with new growth appearing as multiple buds or “witches broom”. Necrotic-corky areas on the fruit surface may sometimes occur in extreme situations. In some cases fruit cracking occurs with exudates.
Copper deficiency is more likely to occur in new plantings on previously uncropped soils, which are usually deficient or totally lacking in copper. In California, it has been referred to as “corral” disease or “midden” disease because it is associated with high organic matter that ties up the copper, or old Native American sites were debris had been piled. It is often localized in certain areas. Once I saw it on nursery trees that had had inadequate copper in the nutrient solution. I've only seen it on citrus, and not any other subtropical like avocado, but that doesn't mean it can't happen.
I've also seen gummosis similar to this occurring with drought and water management. It more commonly occurs as a twig die back at the tips. And certainly Phytophthora gummosis will show gumming. It's that little gumming pocket under the bark is usually the way to distinguish copper deficiency from these two others.
Pictures: pocket gumming (U. of Florida), oozing (Yara), gumming, and more gumming not a worm
- Author: Jeannette E. Warnert
A tell-tale sign of spring in California is a flush of new leaf growth on citrus trees. Because the feathery light green leaves are particularly attractive to Asian citrus psyllids (ACP), the leaves' emergence marks a critical time to determine whether the pest has infested trees.
“We encourage home citrus growers and farmers to go out with a magnifying glass or hand lens and look closely at the new growth,” said Beth Grafton-Cardwell, UC Agriculture and Natural Resources (UC ANR) citrus entomologist. “Look for the various stages of the psyllid – small yellow eggs, sesame-seed sized yellow ACP young with curly white tubules, or aphid-like adults that perch with their hind quarters angled up.”
Pictures of the Asian citrus psyllids and its life stages are on the UC ANR website at http://ucanr.edu/acp. If you find signs of the insect, call the California Food and Agriculture (CDFA) Exotic Pest Hotline at (800) 491-1899.
Asian citrus psyllids are feared because they can spread huanglongbing (HLB) disease, an incurable condition that first causes yellow mottling on the leaves and later sour, misshapen fruit before killing the tree. ACP, native of Pakistan, Afghanistan and other tropical and subtropics regions of Asian, was first detected in California in 2008. Everywhere Asian citrus psyllids have appeared – including Florida and Texas – the pests have found and spread the disease. A few HLB-infected trees have been located in urban Los Angeles County. They were quickly removed by CDFA officials.
“In California, we are working hard to keep the population of ACP as low as possible until researchers can find a cure for the disease,” Grafton-Cardwell said. “We need the help of citrus farmers and home gardeners.”
Grafton-Cardwell has spearheaded the development of the UC ANR ACP website for citrus growers and citrus homeowners that provides help in finding the pest and what to do next. The site has an interactive map tool to locate residences and farms that are in areas where the psyllid has already become established, and areas where they are posing a risk to the citrus industry and must be aggressively treated by county officials.
The website outlines biological control efforts that are underway, and directions for insecticidal control, if it is needed. An online calculator on the website allows farmers and homeowners to determine their potential costs for using insecticides.
There are additional measures that can be taken to support the fight against ACP and HLB in California.
- When planting new citrus trees, only purchase the trees from reputable nurseries. Do not accept tree cuttings or budwood from friends or relatives.
- After pruning or cutting down a citrus tree, dry out the green waste or double bag it to make sure that live psyllids won't ride into another region on the foliage.
- Control ants in and near citrus trees with bait stations. Scientists have released natural enemies of ACP in Southern California to help keep the pest in check. However, ants will protect ACP from the natural enemies. Ants favor the presence of ACP because the psyllid produces honeydew, a food source for ants.
- Learn more about the Asian citrus psyllid and huanglongbing disease by reading the detailed pest note on UC ANR's Statewide Integrated Pest Management website.
- Assist in the control of ACP by supporting CDFA insecticide treatments of your citrus or treating the citrus yourself when psyllids are present.
- Support the removal of HLB-infected trees.