LIGHTNING AND DECLINE OF CITRUS TREES IN

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1 8 FLORIDA STATE HORTICULTURAL SOCIETY, 972 sufficient cold temperatures have occurred to induce cold hardiness in citrus. The lack of hardiness-inducing temperatures during this study did not permit adequate comparison of the 2 threshold temperatures. Under conditions of a more typical Florida winter identification of cold-hardiness inducing periods might be better defined by one or the other threshold temperature. In this study the accepted threshold temperature of 55 was found satisfactory. Although 'Temple' orange is considered to be a less cold hardy variety than 'Hamlin' orange by published ob servational data, this and other research with leaf-freezing points indicates that 'Temple' and 'Hamlin' orange can acquire similar degrees of hardiness if adequate pre-conditioning tempera tures are experienced. Citrus cold hardening temperatures of 55 and 5 occurred more fre quently at Gainesville, least frequently at Dun dee, and with intermediate frequency at Leesburg. The LFP values increased in a highly significant manner as hardiness-inducing tem peratures decreased from Gainesville to Dundee. Literature Cited. Cooper, W. C Cold hardiness in citrus as re lated to dormancy. Proc. Pla. State Hort. Soc. 72: Cooper, W. C, B. S. Gordon and S. D. Tayloe. 95. Freezing tests with small trees and detached leaves of grapefruit. Proc. Amer. Soc. Hort. Sci. 63: , Jack Hearn, Gordon K. Rasmussen, and Roger H. Young The Florida freeze. Calif. Citrograph. 8: Gerber, J. F. and F. Hashemi The freezing point of citrus leaves. Proc. Amer. Soc. Hort. Sci. 86: Hearn, C. J., et al Influence of variety and rootstock upon freeze injury to citrus trees in the 962 Florida freeze. Proc. Fla. State Hort. Soc. 76: Hendershott, C. H The responses of orange trees and fruits to freezing temperatures. Proc. Amer. Soc. Hort. Sci. 86: Hutcheson, C. E. 97. Leaf freezing points and cold hardiness of citrus. Master's Thesis. Univ. of Fla., Gaines ville. 8., and W. J. Wiltbank. 97. Cold hardiness of selected citrus varieties as determined by freezing de tached leaves. Fla. State Hort. Soc. 83: , and 972. The freezing point of detached leaves as a measure of cold hardiness of young budded citrus plants. HortScience. 7() : Jackson, L. K. and J. F. Gerber Cold tolerance and freezing point of citrus seedlings. Proc. Fla. State Hort. Soc. 76:7-7.. Lindsey, R. J. 97. Environment and citrus cold hardiness. Master's Thesis. Univ. of Fla., Gainesville. 2. McCown, J. T Field observations of Florida citrus following the freezes. Proc. Fla. State Hort. Soc. 7: Sites, J. W. 96. Preconditioning of plants in rela tion to cold tolerance. Fla. Agr. Exp. Sta. Ann. Rept. 96: 2.. Young, Roger Cold hardening in citrus seed lings as related to artificial hardening conditions. J. Amer. Soc. Hort. Sci. 9(6): Young, R. H., and A. Peynado. 96. Effects of artificial freezing on Red Blush grapefruit. J. Rio Grande Valley Hort. Soc. 5: , and A. Peynado Growth and cold hardiness of citrus and related species when exposed to different night temperatures. Proc. Amer. Soc. Hort. Sci. 8: LIGHTNING AND DECLINE OF CITRUS TREES IN E. P. DuCharme FLORIDA IF AS Agricultural Research and Education Center Lake Alfred Abstract. Each year lightning either direct y or indirectly causes considerable tree loss in Florida citrus groves. One to many trees may be affected by one or more lightning strikes in any grove at one time. The number of trees affected by one strike depends on the size of the discharge area and the planting distance of the trees. The largest number of trees known to have been killed by lightning during one storm was,77 trees in 9 acres of a flatwoods grove. Usually, or 2 trees may be killed when the strike occurs and others will be affected to a Florida Agricultural Experiment Station Journal Series No. 65. GROVES lesser degree progressing from the center of the strike to the periphery of the discharge area. Frequently, some of the more severely affected trees near the center of the strike area decline gradually and die from what appears to be root rot, heart rot, or foot rot within to 3 years after the strike. The less affected trees on the periphery of the discharge area generally do not decline or die. The secondary effects of lightning strikes on citrus trees may have the appearance of a disease spreading through the grove. The symptoms of lightning injury differ from those of rough lemon decline and other declines; but when lightning injury is superimposed on some other condition, the causes of the decline and loss of trees may be obscure. Lightning* is a spectacular and frequent phe nomenon in citrus groves of Florida. Some of the effects of lightning on citrus trees were

2 DUCHARME: LIGHTNING INJURY 8 recognized and described by Stevens in 98 (, 2, 3). Later lightning was determined to be the cause of decline and loss of trees in.% of the groves surveyed for spreading decline caused by the burrowing nematode, Radophohis similis (Cobb) Thorne 99 (). Each year, lightning either directly or indirectly continues to be the cause of decline and loss of many trees in Florida citrus groves. Sometimes the effect of lightning on citrus is clear and precise, but frequently its effects are quite subtle and are not immediately apparent. Often the effects of lightning injury are slow to appear and decline develops long after the strike has occurred, and the storm that produced the discharge has been forgotten. Then the most recent disease to plague citrus or perhaps a newly introduced pesticide, herbicide, and even fertilizer materials might be blamed for the condition. It is even commonplace to have light ning damage confused with the decline and loss of trees caused by tristeza, spreading decline, root rots, flood damage, and more recently by Young Trea Decline and Sand Hill Decline fre quently called rough lemon decline. Because this confusion can and does happen, it was deemed appropriate to present information regarding the kinds and extent of damage caused by lightning in Florida citrus groves and compare the characteristics of lightning injury and rough lemon decline. Characteristics of Lightning Injury in Citrus The effects of lightning on citrus are not characterized by physical violence as often hap pens when lightning strikes other kinds of trees. There is no obvious mechanical damage to citrus at the time of the strike and no fires are started. Branches are not wrenched from the tree and bark is not torn from the trunk or branches by the force of the discharge. Sometimes inconspicious narrow slits may be observed in patches of killed bark on large branches, trunk, and crown roots. These slender splits in the bark are not evident for several days after the strike has occurred. Usually, the first symptoms that become evi dent are permanent wilting of the foliage, yellow ing of the midveinis and leaf blades, and de foliation. Wilting of leaves may start within to 6 hr. after the strike. Leaves of affected trees abscise between the petiole and blade. Peti oles usually remain attached for a while but eventually drop. Death of the foliage may be so rapid that it dries, turns brown, and may re main attached for several weeks. Such trees have the superficial appearance of having been damaged by fire or cold. These foliar symptoms may occur on all or only a portion of the tree depending upon the intensity of the discharge and how much of the tree was injured. One of the most characteristic symptoms to appear on trees struck by lightning is the injury to young green shoots and twigs. Small round to large irregularly shaped patches of green bark between the buds are killed and become tannish gray to brown while the bark surround ing the buds is apparently not damaged and remains green. These irregular shaped patches of injured bark may girdle the small stems or twigs, coalesce into larger areas but the buds remain green and isolated by the brown dead bark. The green bud surrounded by dead bark is a transitory symptom and appears only on the small green shoots and twigs where the discharge occurred. This symptom is evident for only a few weeks and the green disappears as the injured twigs and stems die. This "greenbud-island" symptom has not been observed on tress affected by any other disorder and may be considered specific for.lightning injury. Injured terminal twigs most commonly occur at the top of the tree but may be observed at any place on the periphery of the canopy, even on the tips of branches in contact with the soil or with the cover crop. Patches of bark may be killed on the trunk, main scaffold, and small branches in the path way of the lightning discharge. Ocassionally, a continuous strip of bark or all of the bark on the trunk and one of the main branches will be killed from below the soil surface to the top of the tree. Within a few days, the injured bark develops a water-soaked appearance and becomes somewhat darker than the unaffected adjacent bark. Wood and inner bark in the affected areas become somewhat yellow and gum may begin to accumulate between the wood and injured bark. This gum deposit may account for some of the yellow discoloration. The affected bark is softer than the healthy bark and gradually becomes more granular as it dries and blackens. The blackened bark crumbles readily when rubbed, splits longitudinally as the disintegration and drying proceeds, separates from the wood, and often falls from the tree. The size of the dead patches and total amount of bark killed in struck trees varies with the intensity of the discharge. Injured bark and wood on the trunk and

3 82 FLORIDA STATE HORTICULTURAL SOCIETY, 972 branches become points of entry for wood borers such as shot hole borers and flat headed borers. Several species of Scolytids commonly known as "shot hole borers" invade bark and wood killed by the lightning discharge. Shot hole borers usually enter into killed tissues within 8 to days after the strike has occurred; but if pene tration takes place, a few days after the strike, a small drop of gum will accumulate on the surface of the bark over the entrance hole. The areas of bark and wood killed by the discharge are detected frequently by slender cylinders of white to gray wood detritus protruding from the small holes made by shot hole borers as they penetrate. These slender columns of detritus are approximately /6 inch in diameter and up to /2 inch long. A flat headed borer, Chrysobothris chryseola, commonly invades and feeds only on dead inner bark, phloem, and cambium. The roots of trees struck by lightning may be killed, severely damaged, or not affected, depend ing upon the intensity of the discharge. If the damage is severe, death of the roots is immedi ate and bark decay and disintegration take place rapidly. Bark on killed roots soon becomes soft, water-soaked, turns brown and granular, and finally black. If the discharge strike is relatively mild and only weakens the tree, then root rot develops slowly and such trees may either die of root rot to 3 years later or may recover. Another symptom frequently observed on trees struck by lightning is a band of injured bark to 2 inches wide just under the soil surface. This band of dead bark is often present even though no other bark damage can be detected on the tree or trees struck in the discharge area. This narrow band of killed bark may be made up of a series of small areas on one side of the tree or may completely girdle the trunk as a continuous band of dead bark. If the damage is not extensive or too severe, new wood and bark may be regenerated over the injuries and the trees recover. Exposed wood may also become infected by wood invading organisms such as Ganoderma spp. which cause further trunk de struction. Trees in the later stages of lightninginduced trunk decay might be erroneously diag nosed as initially caused by wood rot organisms. Long range effects on lightning struck trees become evident only or more years after the discharge occurred. Trees mildly to moderately injured by the discharge may be weakened and predisposed to secondary infection by bark and wood rot organisms. These trees may decline and die at first in the center of the strike then pro gressively towards the periphery of the dis charge area. When this happens, the decline and loss of these trees could easily be interpreted as a disease spreading in the grove. Not all trees in a strike area are visibly damaged and some may be only slightly injured but suffer little or no permanent damage. For example, a 5-acre block of 35 trees 2 years old was struck and every tree had at least one green shoot with typical symptoms of lightning injury, i.e., green buds and dead bark. No trees were killed at the time and none were reported to have died during the next years. These trees recover from the shock and perhaps may be struck again years later. It is, therefore, possible that lightning could strike the same tree or trees more than once. Lightning can strike in groves at any time during the year when thunder storms occur. Ordinarily most strikes take place during the summer and infrequently during spring and fall. Known strikes affecting many trees in blocks of or more acres occurred during August in flatwoods areas. Five unusually large strikes have been recorded during August during the past 3 years. The number of trees struck by one strike depends on the size of the discharge area and on the tree spacing. The recognized number of trees struck in groves during any one storm has varied from one to more than 3,. Usually, less than 2 trees are affected in discrete dis charge areas with only to 3 trees killed or several damaged. The number of strike areas in a grove during a storm likewise may vary from one to several depending on the frequency and intensity of the lightning discharges. The size of known discharge areas can and does vary from one tree space to as much as acres. The largest lightning strike known up to the present in one grove during one storm in volved 3 acres in 3 contiguous -acre blocks of 9 trees each. Three weeks after the strike,59 trees were dead or severely damaged and G v/eeks later 3,296 were either dead, severely damaged, or had symptoms of lightning injury, Table. According to the groupings of trees injured or killed in these blocks, there were 3 discharge centers, Table. Affected trees were also noted in other blocks immediately adjacent to the blocks where most of the damage was concentrated. The shape of the discharge areas as reflected by the distribution, of killed and injured trees is extremely variable. The common shape of strike

4 DUCHARME: LIGHTNING INJURY 83 Table. Number of trees years old killed or injured by lightning in 3 blocks of acres each in a flatwoods grove during a storm in mid-august, weeks after storm 6 week s after storm Trees dead or in Trees Advanced Twig Total affected Block No. advanced- decline dead decline injury No. Percent Total,59 936,69,25 3, or discharge areas is either an irregular circle of oval or it may be somewhat pear shaped. Occasionally, the area involved may be either lenticular or half moon shaped. A large and very unusual distribution of injured trees in approxi mately acres of grove is illustrated in Fig.. Citrus tree damage due to lightning strikes has been found in all areas of the State where citrus is grown. It is difficult to determine where most of the strikes occur because not all strikes are detected and only relatively few are reported. Groves where the largest strikes have occurred recently are mostly flatwoods groves particularly in the Arcadia, La Belle, and Immokalee areas. Trees along roads, wide middles, and edges of groves next to open areas seem to be hit more frequently than the interior of the groves. Perhaps this is only apparently so because these trees are seen most frequently. Comparison of Lightning Damage and Rough Lemon Decline. Lightning strikes, like rough lemon decline, may occur at any place in a grove. 2. There is no spread from tree to tree of de cline caused by lightning either within or outside the lightning discharge area. Only trees actually struck by lightning decline and die within local ized areas. Rough lemon decline gradually spreads throughout the blocks or groves of susceptible trees until all of the susceptible trees become affected. 3. Lightning strikes trees of all ages from those in nurseries to mature trees more than Fig.. A lightning discharge area in a flatwoods grove of trees 7 years old. The strike or strikes took place during a storm, August 7, 97, and the photograph was taken November 23, 97 after the killed trees had been pulled.

5 8 FLORIDA STATE HORTICULTURAL SOCIETY, years old. Rough lemon decline on the other hand has been observed to affect only trees that are more than 5 years old.. Trees that decline and die from lightning injury commonly develop root rot and perhaps foot rot, whereas rough lemon decline does not. 5. Not all trees struck by lightning die. Sus ceptible trees on rough lemon rootstocks have not been known to permanently recover once they have developed symptoms of rough lemon decline. When lightning and rough lemon decline simultaneously affect trees in a block or grove, the decline and death of trees can be spectacular. When this happens, the cause or causes of the decline may not be readily apparent and may lead to confusion as well as consternation. There seems to be no way to avoid lightning damage in citrus groves other than possibly equipping each tree with lightning rods. It can be expected that lightning will continue to cause considerable tree loss each year; and in some years, there will be more damage than in other years. There seems to be no prospect that these losses will diminish in the future, especially in the flatwoods areas. Literature Cited. Rhoads, Arthur S., and E. F. DeBusk. 93. Diseases of citrus in Florida. Univ. Fla. Agr. Expt. Sta. Bui p Stevens, H. E. 98. Lightning injury to citrus trees in Florida. Phytopathology 8: Florida citrus diseases. Univ. Fla. Agr. Expt. Sta. Bui. 5. p Suit, R. F., and E. P. DuCharme. 97. Citrus de cline. The Citrus Ind. 28(7) :5-8, 3.