CAUSE AND NATURE OF SPREADING DECLINE

Transcription

1 DUCHARME: CAUSE OF SPREADING DECLINE 75 CAUSE AND NATURE OF SPREADING DECLINE OF CITRUS E. P. DuCharme Florida Citrus Experiment Station Lake Alfred As Dr. Camp has just told you, spreading decline is the most important trouble affecting citrus groves in Florida today. The primary cause of this disease has been established as the burrowing nematode, Radopholus similis (Cobb) Thorne. Spreading decline was first noticed in Florida before 1930, but it was not until 1953 that the true cause of the disease was demonstrated. It is my purpose here to discuss for you the research that led to the discovery that the burrowing nematode causes spreading decline; to describe the disease and show how it can be differentiated from other types of decline in Florida citrus groves; to tell you something about the nature of the dis ease and how it operates; and to discuss briefly other plant parasitic nematodes found in citrus groves. A typical spreading decline area consists of a group of trees all of which have the same non-thrifty appearance. The trees are stunted, have undersized leaves, sparse foliage, reduced terminal growth, lowered yields and exten sive deterioration of the feeder root system below a depth of about 20 inches. Such trees remain in a non-thrifty condition indefinitely but are not killed by this disease. Because of their deficient root system, diseased trees al ways wilt more readily during periods of drought than the adjoining healthy trees. They frequently show temporary improvement under favorable moisture conditions. The de cline area is usually sharply separated from the healthy trees in the remainder of the grove, and the area may occur at any point in a grove. What distinguishes it from other declines is the fact that the area spreads con tinuously and about equally in all directions regardless of elevation or direction of rows and cultivation. This constant spread in all directions is one of the most characteristic features of the dis ease and led to its name, "spreading decline." For years the measurable annual rate of spread was the most reliable characteristic for diagnosing spreading decline in a grove. The rate of spread of the decline condition to new trees varies from grove to grove and from year to year in the same grove, but the aver age rate of spread in 25 groves for 5 years was 1.6 trees per year on the margin, as re ported by Suit and Ford (13). From centers of infection, spreading decline moves out in all directions in a grove and crosses wide mid dles into other properties. So far, spreading decline has successfully crossed clay roads, asphalt roads with rights-of-way up to 100 feet wide and even railroad lines. Bridging these distances between groves is probably accom plished by root contact or near contact. At the edges of groves, the roots of rough lemon stocks have been traced outward for as much as 50 and 60 feet. The characteristics of spreading decline as outlined above are suffi cient to differentiate it from other types of decline such as foot rot, water damage and psorosis, which in some cases may produce trees of the same appearance but which do not spread in the same way. There are certain measurable physiologic effects of this disease on affected trees. Ford (5 and 6) established that the respiration rate and catalase activity is lower in the roots of decline trees than in those of healthy trees and higher in roots of the first and second trees from the margin than it is in either the de cline area or in the healthy trees more than three rows beyond the disease margin. Once the condition known as spreading de cline was recognized, efforts were immediately started to find the cause so that satisfactory control measures could be devised. The first lines of attack on the problem were naturally directed toward what appeared to be the most obvious factors that might have been responsi ble for the condition. As these early studies did not lead to a solution of the problem, the work was gradually expanded and intensified along many lines of research so that all prob able factors were being investigated at the same time. The general non-thrifty aspect of declining trees suggested that nutrition might be a pri mary causal factor. Consequently the nutri tion of the affected trees in these areas was thoroughly investigated. Many experiments were designed to improve the fertilization

2 76 FLORIDA STATE HORTICULTURAL SOCIETY, 1954 practices in the areas of decline in an effort to correct the non-thrifty condition and to obtain evidence that would possibly demonstrate un suspected minor element deficiencies. None of these was successful. It was concluded that the trouble was not primarily nutritional. An intensive study was started to discover whether a fungus could be causing the dis ease, since the pattern of spread seemed like that of a fungus. In all, more than 30,000 cultures of microorganisms (fungi, actinomycetes and bacteria) were isolated by Voorhees, Suit, Bliss, Sherbakoff, DuCharme and others from the rootlets of declining and healthy trees. More than 60 species of fungi and an undetermined number of actinomycetes and bacteria were isolated, but the same kinds of microorganisms were found associated with rootlets of both decline and healthy trees in about the same numbers and no one kind of microorganism was found associated only with the areas of spreading decline. Hundreds of rough lemon seedling test plants were inocu lated with isolates of the most commonly found microorganisms, mainly fungi. None of the fungi or other microorganisms tested caused typical symptoms of spreading decline under controlled conditions. Reports on the results of some of these studies have been made by Suit et al. (10, 13 and 14) and Sherbakoff (8). Further investigations made since then confirm these observations. Almost all the fungus studies were made on feeder roots because there is virtually no destruction of secondary roots and none of the fungi iso lated from feeder roots was ever isolated from the interior of roots 1/8 of an inch in diameter or larger. Along with the fungus studies, experiments were made to determine whether or not a virus could be the cause of spreading decline. Four types of grafts were tried in this part of the investigation, between 1945 and More than 3000 buds from trees affected by spreading decline were budded into healthy test trees, including seedlings, budded nursery trees and mature grove trees. During this same period, close to 900 pieces of roots from decline trees were grafted into the root sys tems of healthy trees, both young and old. Numerous healthy trees were also patchgrafted with immature leaf tissue from de cline trees. In 1951, a unique experiment was tried. Two rows of six trees, each pass ing from a spreading decline margin into the adjacent healthy area, were connected to gether by means of 20 root grafts between each pair of trees, with the idea that if it was a virus, the rate of spread would be increased in these rows, but this was not the case. Spreading decline was neither transmitted nor rate of spread increased by any of these pro cedures, yet some of the experimnts have been in progress for nine years. All of the evidence obtained by these methods indicates that spreading decline is not a virus disease. Re ports on some of these experiments were made by Suit (10) and Suit and Knorr (14). Efforts were made by Dr. Wander (unpub lished data) to demonstrate toxic substances possibly produced by soil organisms which did not directly attack the trees-these organisms being present in the soil in areas affected by spreading decline but not present in healthy areas of the same grove. If a substance could be found that would be harmful to tree growth, it could be considered as a possible cause of spreading decline. Accordingly, soils from decline and healthy areas were leached with various solvents and in all cases the same kinds of substances were extracted and iso lated from these areas. Depressing effects on citrus seedlings under controlled conditions were obtained with some of these substances, but the concentrations required to inhibit seedling growth was far in excess of the amounts present in the soil. It was finally concluded that soil toxins were not the cause of spreading decline. Studies on plant parasitic nematodes that could possibly be associated with spreading decline were started by Suit (10) in Since citrus nematodes, Tylenchulus semipenetrans Cobb, had been found parasitizing citrus roots in Florida (1), the first phase of these studies was to determine whether or not the citrus nematode could be the cause of spreading decline. Study of this nema tode in relation to spreading decline in dicated that it was not the cause of the dis ease, but the possibility that some other nema tode could be involved was suggested by Suit and Knorr (14). Further investigation pro duced evidence that the total nematode popu lation associated with the roots of trees in de cline areas was larger than that associated with the roots of healthy trees in the same grove. The abnormal deterioration of fibrous roots of trees affected by spreading decline was ini-

3 DUCHARME: CAUSE OF SPREADING DECLINE 77 tially reported by Suit (10). Since the pri mary cause of spreading decline could be ascribed neither to a fungus nor to the citrus nematode, more detailed studies were made on the origin of rootlet deterioration and the extent of rootlet destruction. Root observa tion boxes were constructed in the field and in the laboratory greenhouse in order to study the origin and development of brown lesions detected on rootlets of declining trees. A de tailed study to measure the reduction of feeder roots and to determine the zone of greatest rootlet loss, made by Ford (5), demonstrated that decline trees had about 40 percent less feeder roots than healthy trees and that most of the destruction occurred below 20 inches. At depths below 30 inches, decline trees have 90 percent less feeder roots than healthy trees. Following these observations, a critical study of total nematode population yielded positive evidence that more nematodes were associated with the rootlets of decline trees than with the roots of healthy trees at depths below 20 inches. An immediate and intensive search was made to find a nematode common to areas of spreading decline but not present in the healthy parts of the same grov<\ This investigation by Suit and DuCharme (12) led to the discovery in 1953 that the burrowing nematode, Radopholus similis (Cobb) Thorne, is the primary cause of spreading decline. The proof that the burrowing nematode is the cause of spreading decline was established by the following observations and experi ments: (1) The burrowing nematode was found to be the only parasitic nematode associated with the lesions in the feeder roots of spreading de cline-affected trees. A study of the feeder roots of affected trees showed that there were many brown lesions in the rootlets but that very few or no such lesions occurred on the rootlets of healthy trees. Since no fungus pe culiar to spreading decline affected trees could be found associated with these lesions, the rootlets were dissected and a search was made for an endoparasitic nematode. These dissec tion studies revealed that in the discolored areas of the cortex there were small cavities connected to the exterior of the rootlet by tiny holes and splits in the epidermis. Bur rowing nematodes were found in these cavities and tiny tunnels, within the discolored cortex tissues surrounding the cavities and in the healthy cortex in advance of the discolored areas. The xylem or woody tissue of the root lets was not penetrated by this nematode. Rootlets from eleven groves with known cen ters of spreading decline were studied in this way. The burrowing nematode was found only within the feeder roots of trees from the spreading decline areas and in the feeder roots of the first apparently healthy tree, but never from rootlets of healthy trees outside the in fested area. The burrowing nematode was also found in the soil about the rootlets of trees in areas of spreading decline but not in the soil about the roots of healthy trees away from the infested areas. (2) In citrus groves, the burrowing nema tode occurred only in close proximity to trees affected by spreading decline. Following the root dissection study, a survey was made in fifty groves in order to see whether or not the presence of the burrowing nematode was cor related with known centers of decline. Utiliz ing the sieve and Baermann techniques for ex tracting nematodes from soil and root samples, the burrowing nematode was found in all the decline areas but not in healthy portions of the same groves, samples from healthy trees being collected at least 200 feet beyond the last vis ibly affected trees. In addition, the burrow ing nematode was not found in samples col lected in 26 groves where there was no evi dence of spreading decline. (3) Reproduction of spreading decline symptoms on test trees under controlled con ditions was achieved with naturally infested subsoil and hand picked specimens of the bur rowing nematode. Since the zone of greatest rootlet deterioration was found to be below 18 inches, naturally infested subsoil was collected from a depth of two feet from four distinct centers of spreading decline. A second series of samples was also collected from a depth of two feet in the healthy part of the same groves at least 200 feet ahead of the margin of the diseased area. These subsoil samples were collected early in the morning and carefully protected during transportation so that the temperature of the samples did not rise. The soil, upon arrival at the laboratory, was imme diately placed in five-quart cans. One sour orange seedling six inches high was planted in each can. There were six replicates of plants in subsoil from the decline area and six in sub soil from the healthy area of each of the four groves sampled. In all, there were 24 test

4 78 FLORIDA STATE HORTICULTURAL SOCIETY, 1954 plants growing in naturally infested subsoil and 24 check plants. The cans containing the soil and test plants were kept in a water bath at a constant tem perature of 75 F. to 78 F. This tempera ture was selected because thermograph rec ords taken at a depth of three feet in a typical spreading decline area showed that the tem perature at that depth was between 69 F. and 79 F. for approximately one-half of the year, while a temperature of 80 F. was fav orable for citrus root growth. Six weeks after the start of this experiment, it was evident that the seedlings growing in the "decline" subsoils were not growing as well as the check plants in subsoil from the healthy parts of the groves. The experiment was continued for six months. At that time, all the plants grow ing in the "decline" subsoil (from the four groves) were stunted and there was extensive deterioration of the rootlets. The same kind of brown lesion observed on the rootlets of naturally infested trees were also present on the rootlets of these test plants. Dissection of the rootlets revealed the burrowing nematode in the rootlets of the test plants but not in the rootlets of the check plants. This was true for all plants grown in infested soil. No other plant parasitic nematode was found either in the rootlets or in the soil about the roots of any of the 48 plants in this experiment. In another test, 12 plants were grown in fumigated soil and the roots of six of them were infested with 200 adult burrowing nematodes picked out under the microscope. Three months later the test plants were stunted and the rootlets were extensively deteriorated, whereas the check plants were making good growth and showed no symptoms of decline. An examination of the soil and rootlets showed that burrowing nematodes were present in the soil and within the roots of the test plants only. Not only were they present, but their numbers had increased materially and larval stages were found in abundance. This dem onstrated that the burrowing nematode is able to reproduce within citrus roots. No other plant parasitic nematodes were present in the roots of these plants. At this point, it should be mentioned that during the early stages of the work, the dis ease could not be reproduced under controlled conditions in pots either in the greenhouse or in the open, using either soil from affected groves or soil inoculated with affected roots. This greatly hampered the progress of the work until it was found that the focus of the disease appeared to be below a 20-inch depth and that below this depth feeder roots were formed but disintegrated within a week, leav ing the bare laterals. This led to experi ments in which the temperature of the soil was controlled at about 75 F., in which case there was a rapid decline of plants growing in subsoil from affected areas. This led to the building of large tanks in which pots of soil could be maintained between 75 and 78 F. Under these conditions, it was easy to produce the decline using soil from decline areas. This was one of the really big ad vances in the course of the research and these tanks are now serving for the screening of rootstocks and chemical treatments as well as for direct work on the nematode, itself. The three original tanks each contained 108 cans, each can was connected to a drainage system, and now a new and larger type is under con struction. The burrowing nematode was first found and described by Cobb in 1890 on the roots of bananas in the Fiji Islands (3). Subsequently it was found attacking many other plants in widely separated countries. A discussion of the known distribution and host range of this nematode is given by Brooks (1). The bur rowing nematode is known as Radopholus similis (Cobb) Thorne 1949, and its past syn onymy is as follows: Tylenchus similis Cobb 1893, Tylenchus acutocaudatus Zimmerman 1898, Tylenchus biformis Cobb 1919, Anguillulina similis (Cobb 1893) Goodey 1932, and Rotylenchus similis (Cobb 1893) Filipjev The burrowing nematode is a small plant parasitic worm about 1/50 of an inch long. It requires a source of living plant tissue for its food supply and feeds on the tender cortex tissues of rootlets, forming burrows and cavi ties. The females deposit eggs inside the root lets where the young are hatched. The young feed on the rootlets until the available food supply is exhausted and then leave the deteri orated rootlet in search of sound roots suitable for food. The burrowing nematode spends most of its life within rootlets and is therefore classified as endoparasitic in contrast to para sitic nematodes that live outside the root in the soil and classified as ectoparasitic. When it leaves the deteriorated rootlet and moves about in the soil, it is in a migratory phase.

5 DUCHARME: CAUSE OF SPREADING DECLINE 79 The duration of the migratory period depends on the length of time required to find and en ter another healthy rootlet. It is in this migra tory stage that the nematodes, traveling in all directions in search of roots, extend the area of infestation and thus account for the "creep ing" nature of spreading decline. The actual distance that an individual burrowing nematode may travel in the soil has not been de termined, but it is probably not very far. Bur rowing nematodes have been found invading citrus rootlets 12 feet deep. They have been found in soil and roots of trees two or three rows in advance of the visible disease margin. From the known average annual rate of spread of the disease and the lateral spread of these nematodes beyond the visible diseased area, it becomes apparent that trees are infested one to two years before showing symptoms of de cline. The lack of an efficient method for easily extracting burrowing nematodes from samples was one of the main reasons why it was so difficult to find the parasite the first time and to study the disease. Root dissec tion under a microscope, though a reliable method for finding the nematode, is slow, tedi ous and not adapted to intensive research on the nature of the disease, control studies and extensive survey work. Before studies of this nature could be intensified, a satisfactory method had to be devised to separate easily and positively burrowing nematodes from sample material. Methods commonly used for the isolation of other plant parasitic nema todes were not satisfactory in these studies and it was often necessary to sample repeat edly a suspected area or even known infested areas in order to find burrowing nematodes. This failure to extract the nematodes from samples in which they were present inter fered with the progress of investigation. A much simpler method was devised in coopera tion with Dr. H. W. Young of the Sub-Tropical Experiment Station, who was investigating the burrowing nematode on avocados. The root sample is carefully washed and stored for a few days in a closed pint jar. At the end of this incubation period, the roots are washed with about 2 teaspoons of water to remove the nematodes that have crawled out of the rootlets, and this wash water containing the nematodes is examined under the microscope. This technique assures the investigator of find ing burrowing nematodes if they are present in the sample. Not only is the method easy to use, but it is highly efficient for processing a large number of samples with certainty; it thus expedites the survey and research work on spreading decline. The dissemination of spreading decline be comes understandable when the knowledge that it is caused by the burrowing nematode becomes available. Infested nursery stock used for setting out new groves or for re plants in previously non-infested groves ac counts for establishment of new centers of in fection. Some of our common ornamental plants are hosts of the burrowing nematode; this explains why so often spreading decline has started adjacent to homesites and buildings in and about groves. There is still no evidence to date that machinery is a factor in the trans mission of buitowing nematodes, but infested roots could conceivably become entangled in machinery and subsequently be deposited in new locations, where, if conditions were favor able, the nematode could become established. Therefore machinery used for working soil should be cleaned of all plants parts as a precautionary measure before moving such equipment from decline groves. Other Nematodes In the process of this research, nine other species of plant parasitic nematodes not hith erto reported on citrus in Florida have been found associated with citrus roots. In addi tion to the citrus nematode which is well known, the sting nematode was found by both the Station workers and Dr. Christie, and the ring nematode which had been previously re ported from Florida by Dr. Steiner (9) was found repeatedly in the course of this work. Of the nine parasitic nematodes found, three had been reported associated with citrus roots in other regions and six had never been reported associated with citrus. Although all of these have been found occasionally in areas of spreading decline, none of them is either common to or confined to areas of spreading decline. Outside of the citrus nematode and the burrowing nematode, the type and extent of damage that may be caused by these nema todes has not been established either here or elsewhere and some of them may do little or no damage and others may be important and produce other types of decline. Investiga tions along this line are being pursued as rap idly as time and facilities permit. Pertinent information concerning these nematodes in re-

6 80 FLORIDA STATE HORTICULTURAL SOCIETY, 1954 lation to citrus and their occurrence in Florida citrus groves is given here to familiarize the growers with this phase of the research in progress. The citrus nematode, Tylenchns semi-penetrans, Cobb, was reported on citrus roots in Florida by Byars in 1921 (2). This nema tode is able to cause a non-thrifty condition of trees in severely infested groves but it does not cause the spreading type of decline. Thus far, the citrus nematode has been found in only 13 groves affected by spreading decline, in 22 groves affected by some other type of decline and Suit and Knorr (14) reported finding it in 12 out of 115 apparently healthy groves investigated. The sting nematode, Belonolaimus gracilis Steiner, was reported as causing possible dam age to citrus in Florida by V. G. Perry and J. R. Christie (unpublished data 1953) and a report on its relationship to spreading decline was made by Suit and DuCharme (12). In our investigations, the sting nematode has been found in only 16 of 147 groves affected by spreading decline, in 14 groves affected by some other type of decline, and in 4 appar ently healthy groves. It should be pointed out, with this nematode and other nematodes reported below, that relatively few healthy groves were being investigated for these par ticular nematodes, so the small numbers re ported in this category are not significant. In one experiment, 200 hand-picked adult sting nematodes were placed about the roots of sour orange seedlings growing in soil free of all plant parasitic nematodes. In all cases, the sting nematodes became established and at tacked the roots of the citrus test plants. In one case, 17 nematodes were sufficient to es tablish an infestation around the roots of a test plant. No symptoms of decline were manifested by these test plants within six months. Nevertheless, this study demon strated that sting nematodes are able to re produce and maintain high populations when only citrus roots are available as a food sup ply. The meadow nematode, Pratylenchus pratensis (de Man) Filipjev, has been reported on citrus in Brazil (4), California (15) and Flor ida (11). In the course of the investigations on spreading decline, it has been found in 21 of 147 affected groves, in 13 groves affected by some other type of decline, and in 7 ap parently healthy groves. The meadow nema tode resembles the burrowing nematode but does not cause spreading decline. Studies made in relation to citrus demonstrate that the meadow nematode penetrates the roots of citrus, makes cavities and reproduces there. Further investigation is being carried on to determine the exact importance of this root parasite to citrus. The spear nematode, Hoplolaimus coronatus Cobb, was reported associated with roots of citrus in Florida in 1953 (11). So far, this nematode has been found in 54 citrus gioves; 36 with spreading decline, 13 with some other type of decline and 3 healthy groves. Efforts to infest citrus roots with H. coronatus have not been successful, indicating that this nema tode may not reproduce in citrus roots and that it probably is not of importance to citrus. Aphelenchus avenae Bastian, has been found on citrus roots in Florida (11) and in Rhodesia (7). It has been found in 108 cit rus groves in Florida; 68 with spreading de cline, 33 with some other form of decline and in 4 healthy groves. This nematode is gen erally found only in association with rotting citrus rootlets. It is probably not a factor in spreading decline and probably is not im portant to citrus, since it appears to be feed ing on decaying tissues and fungi. Several other plant parasitic nematodes have been found associated with citrus roots in Florida. Trichodorus sp. Cobb, Hemicycliophora sp. de Man, Criconemoides citri Steiner and Xiphenema americanum Cobb have been reported already (11). In addition to these, Rotylenchus sp. Filipjev, Hexatylus sp. Goodey, and Aphelenchoides sp. Fisher, are here reported for the first time in association with citrus feeder roots. None of these nematodes appears to be a factor in spreading decline and some of them are veiy rarely found on citrus roots. Further study of these nema todes is being continued. In the preceding discussion, I have at tempted to describe for you the principal char acteristics of spreading decline and indicated how it differs from other types of decline in having a constantly advancing border. We have also reviewed the comprehensive re search that led to the discovery that the bur rowing nematode is the primary cause of spreading decline and have outlined the steps

7 BROOKS: BURROWING NEMATODE HOSTS 81 involved, the most important of which were as follows: 1. The burrowing nematode was always found in the decline areas and was not pres ent in healthy areas, whereas a number of other parasitic nematodes were found occa sionally in spreading decline areas and also in other areas where no spreading decline existed. 2. The symptoms of spreading decline have been reproduced under controlled conditions both by using infested soil and by inoculating healthy soil with burrowing nematodes, whereas inoculation with other nematodes and with various fungi did not produce the typical symptoms of the decline. 3. Burrowing nematodes were always re covered from the roots of the inoculated plants and had increased greatly in numbers. 4. Evidence has also been produced to show that the burrowing nematode lives and re produces within the feeder roots of citrus. We consider that this evidence is sufficient to justify the conclusion that the burrowing nematode, Radopholus similis (Cobb) Thome, is the primary cause of spreading decline as described in this discussion. LITERATURE CITED 1. Brooks, Troy L Host range of the bur rowing nematode internationally and in Florida. Proc. Pla. State Hort. Soc. 67. (In press.) 2. Byars, L. D Notes on the citrus nema tode. Phytopath. 11: Cobb, N. A Nematodes, mostly Australian and Fijian. MacLeay Memorial Volume, Linnean Soci ety of New South Wales, pp Filipjev, I. N., and J. H. Schuurmans Stekhoven, Jr A Manual of Agricultural Helminthology. 878 pp. E. J. Brill, Leiden. 5. Ford, Harry W The effect of spreading decline on the root distribution of citrus. Proc. Fla. State Hort. Soc. 65: Ford, Harry W Changes in rate of respi ration and catalase activity associated with spreading decline of citrus trees. Proc. Am. Soc. for Hort. Sci. 61: Goodey, T Soil and fresh-water nema todes. 390 pp. Methuen and Co., Ltd., London; John Wiley and Sons, Inc., New York. 8. Sherbakoff, C. D Fusaria associated with citrus feeder roots in Florida. Phytopath. 43(7): Steiner, G Plant nematodes the grower should know. Proc. of Soil Sci. Soc. of Fla. 4 (B): Suit, R. F Spreading decline of citrus in Florida. Proc. Fla. State Hort. Sotf. 60: Suit, R. F., and E. P. DuCharme. The burrow ing nematode and other plant parasitic nematodes in relation to spreading decline of citrus. Plant Disease Reporter 37 (7): Suit, R. F., E. P. DuCharme. T. L. Brooks, and H. W. Ford Factors in the control of the burrowing nematode on citrus. Proc. Fla. State Hort. Soc. 66: Suit, R. F., and H. W. Ford Present status of spreading decline. Proc. Fla. State Hort. Soc. 63: Suit, R. F., and L. C. Knorr Progress report on citrus decline. Proc. Fla. State Hort. Soc. 62: White, F. A Notes on citrus nematodes. Calif. Citrograph 32 (7): THE HOST RANGE OF THE BURROWING NEMATODE INTERNATIONALLY AND IN FLORIDA Troy L. Brooks Florida Citrus Experiment Station Lake Alfred In our investigations of the burrowing ne matode, Radopholus similis (Cobb) Thorne in relation to the spreading decline of citrus, one important factor to consider is the susceptibility of other plants to the nematode. Such information would be of value in ex plaining the initiation of spreading decline in citrus groves, and especially those adjacent to home sites. If native plants are susceptible, it would also have some bearing on the spread of the nematode from grove to grove. The 1 Synonyms: Tylenchua similis Cobb, 1893; Tylenchus acuttfcaudatus Zimmermann, 1898; Tylenchus biformis Cobb, 1919; Anguillulina similis (Cobb. 1893) Goodey, 1932; Rotylenchus similis (Cobb, 1893) Filip jev efficiency of any type of control for spreading decline in a citrus grove would be affected if susceptible plants were present in adjacent areas. The burrowing nematode occurs in tropical and subtropical regions of the world. It was first observed by Cobb (1) in 1890 as a para site of banana roots in the Fiji Islands. Since that time this nematode has been found in Fiji, Jamaica, the Hawaiian Islands, the Philip pine Islands, Formosa, South India, Dutch East Indies, Java, Brazil, Central America, Louisiana, and Florida, U.S.A. R. similis has been reported as infesting the roots of the following plants: Pineapple, Ananas sativus Schult. (3, 5, 6, 7). Sugar cane, Saccharum officinarum L. (2, 3, 5, 6, 7, 8).