ECOLOGICAL OBSERVATIONS ON TWO LEAFHOPPERS THAT TRANSMIT THE PIERCE'S DISEASE BACTERIUM1

Transcription

1 Proc. Fla. State Hort. Soc. 93: ECOLOGICAL OBSERVATIONS ON TWO LEAFHOPPERS THAT TRANSMIT THE PIERCE'S DISEASE BACTERIUM1 W. C. Adlerz University of Florida, IFAS, Agricultural Research Center, P. O. Box 388, Leesburg, FL Additional index words. Oncometopia nigricans (Walker, Homalodisca coagulata (Say), sharpshooter, citrus blight. Abstract. Oncometopia nigricans (Walker) and Homalo disca coagulata (Say) 2 known vectors of Pierce's Disease (PD) winter as adults on perennial plants in central Florida. In spring many over-wintering leafhoppers leave winter hosts and disperse to spring-summer hosts as the plants become suitable for oviposition. In all, O. nigricans adults were observed on 63 hosts and nymphs on 41; H. coagulata adults were observed on 46 hosts and nymphs on 33. Host plants were both woody and herbaceous. Four O. nigricans host plants and 5 H. coagulata host plants were identified as especially important in the seasonal development of populations and are suggested as candidate plants for study as PD bacterial hosts in Florida. Dispersals after the first and second generations were in April-May and July for O. nigricans, and June and August for H. coagulata. These times correlate with what is known about seasonal natural infectavity of leafhoppers which begins in late April, in creases through July, and continues through the fall. The relationship of Pierce's disease to citrus blight is discussed. Rickettsia-like bacteria (RLB) causing several diseases of woody and herbaceous plants are transmitted by leaf hoppers of the subfamily Cicadellinae (4, 9). Pierce's disease (PD) is an RLB-incited disease that limits grape culture in Florida (10). It has been trans mitted by leafhoppers of all species of the subfamily Cicadellinae tested as a vectors (9) including 2 species that breed on grapevines in Florida: Oncometopia nigricans (Walker) and Homalodisca coagulata (Say) (1). Neither of these insects have approved common names, although, leaf hoppers in the subfamily Cicadellinae are commonly called sharpshooters, and O. nigricans and H. coagulata are locally called the blue and brown sharpshooters, respectively. PD is also transmitted by spittlebugs, family Cercopidae (9). Citrus blight (CB), or young tree decline, is a major deterrent to citrus production in parts of Florida (3). Due to certain similarities in the symptoms of PD of grapevines and CB of citrus and with the knowledge that the PD vectors O. nigricans and H. coagulata are common on both plants, we considered the possibility of an RLB (PD) etiology for CB. To test this, we fed leafhoppers free of PD bacteria on CB affected citrus (Citrus sinensis Osbeck, 'Valencia') in the field and they subsequently transmitted the RLB of PD to grapevine (5). The possibility of an association between PD and CB was strengthened when rough lemon (Citrus jambhiri L.), the rootstock most as sociated with CB occurrence, exhibited dieback symptoms after inoculation with the RLB of PD (7). Although natural transmission of CB by leafhoppers has not been confirmed, some epidemiological studies have been completed on the transmission of PD in Florida by O. nigricans and H. coagulata (1). H. coagulata occurs in the eastern United States from iflorida Agricultural Experiment Stations Journal Series No Florida to North Carolina, and west to Arkansas and eastern Texas (11, 12). It also occurs in Mexico. O. nigricans is distributed throughout peninsular Florida to- an area in northern Florida where it approaches the southern-most distribution of Oncometopia orbona (F), (11, 12). There may be a third undescribed species in a geographic zone between the two (12). O. nigricans cannot be distinguished from O. orbona by external morphological characters. Turner and Pollard (11) listed 11 breeding and 62 feeding host plants for H. coagulata in central Georgia and 18 breeding and 47 feeding host plants for O. orbona. Similar host lists for O. nigricans have not been published. All these species are primarily southern and feed widely on woody and herbaceous plants (1, 11). Ecological studies of O. nigricans and H. coagulata in cluding host plants, breeding habits, and inter-plant move ments were undertaken with 2 main purposes in mind. First, to identify plants that would be most useful in studies of alternate hosts of the PD bacterium in Florida. Second, to relate interplant movements of leafhoppers to what is known about the seasonal buildup of RLB in plants and the occurrence of natural infectivity in leafhoppers. Interest in both PD and CB led us to include cultivated grapevines, weeds in citrus plantings, and citrus itself in the surveys. Materials and Methods Four observation sites were selected that had a wide variety of weeds and some wild Yitis sp. bordering citrus plantings. Four sites were selected especially for observa tion of wild 'Bullace' grape (V. munsoniana Simpson, 'Bullace' special in Fig. 1). Four sites were selected es pecially for observation of leafhoppers on black cherry (Primus seroiina Ehrk.) in the winter. Other weeds were included in observations at 'Bullace' and cherry sites. The citrus and 'Bullace' sites were visited weekly for 2 years beginning January Cherry sites were visited from fall, after leafhopper populations declined in other areas, until the leafhoppers left the trees in the spring. At all sites, 2 observers walked established routes, counting insects on available weeds. Additional leafhopper hosts in the Leesburg area were identified during surveys in June Adlerz and Hopkins published information on the relative abundance of these insects on cultivated grapevine during part of the study period (1). O. nigricans Results Winter hosts. O. nigricans winters in the adult stage on a number of perennial hosts (Fig. 1, 2). Adults are active in warm weather, feeding and making short local flights. Insects inactivated by cold weather often drop to the ground under host plants. Insects revived by increasing temperature during the day resume activities on plants. Usually leafhoppers are most abundant on American elder (Sambucus canadensis L.), blackberry, (Rubus sp.), eastern baccharis (Baccharis halimifolia L.), and black cherry. During the mild winter of 1979 adults were able to survive on lantana (Lantana camara L.), and 'Bullace' grapevines, and they were common on citrus. Copulatory activity and egg masses were observed on American elder in January, 1979, but no nymphs were observed on winter hosts in January or February during either year. Proc. Fla. State Hort. Soc. 93:

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6 aceae: American elder; 3) Ghenopodiaceae: mexicantea; 4) Malvaceae: ceasarweed (Urena lobata L.), teaweeds (Sida sp.); 5) Onagraceae: primrosewillow (Ludioigia peruviana (L.) Hara); 6) Phytolaccaceae: common pokeweed, 7) Rosaceae: blackberry; 8) Rutaceae: citrus; 9) Verbenaceae: lantana. Although wild grape may be the main source of PD bacteria, any plants, especially perennials, are po tentially important RLB hosts. Studies on citrus could be especially significant since RLB causing PD have been transmitted from citrus to grapevine (5), citrus inoculated with PD bacterium de velops decline symptoms (7), and CB is presently the most serious disease of citrus. Literature Cited 1. Adlerz, W. C. and D. L. Hopkins Natural infectivity of two sharpshooter vectors of Pierce's disease of grape in Florida. /. Econ. Entomol. 72: Ball, J. C Seasonal patterns of activity of adult leaf hopper vectors of phony peach disease in north Florida. Environ. Entomol. 8: Childs, J. F. L Observations on citrus blight. Proc. Florida State Hort. Soc. 66: Hopkins, D. L Diseases caused by leafhopper-borne, rickettsialikc bacteria. Ann. Rev. Phytopathol. 17: Hopkins, 1). L., W. C. Adlerz, and F. W. Bistlinc Pierce's disease bacterium occurs in citrus trees affected with blight (Young tree decline). Plant Dis. Replr. 62: Seasonal concentrations of bacterial plugs in grapevines severely infected with the Pierce's disease bacterium. Phytopathology 69:528 (Abstr.). 7. and W. C. Adlerz Pierce's disease bacterium causes a disease of rough lemon citrus. Phytopathology 70:568 Abst. 8. Purcell. A. H Seasonal changes in host plant preference of the blue-green sharpshooter Hordnia circellata. The Pan-Pacific Entomologist 52: Leafhopper vectors of xylem-borne plant patho gens. In: Leafhopper vectors and plant disease agents. Maramorosch, K. and K. F. Harris Eds., pp Academic Press. 10. Stoner, W. N., L. H. Stover, and G. K. Parris Field and laboratory investigations indicate grape degeneration in Florida is due to Pierce's disease virus infection. Plant Dis. Reptr. 35: Turner, W. F. and H. N. Pollard Life histories and behavior of five insect vectors of phony peach disease. USDA Tech. Bull No. 1188, 28 pp. 12. Young, D. A Taxonomic study of the Cicadellinae (Homoptera: Cicadellinae). Part 1. Proconiini. US National Museum Bull 261. Proc. Fla. State Hort. Soc. 93: FLORIDA GRAPES: THE NEXT DECADE R. P. Bates University of Florida, I FAS, Food Science and Hitman Nutrition Department, Gainesville, Florida J. A. MORTENSEN University of Florida, IF AS, P. O. Box 388, Leesburg, Florida T. E. Crocker University of Florida, I FAS, Fruit Crops Department, Gainesville, Florida Abstract. Grapes in Florida are receiving much greater attention than current planted acreage and economic ac tivity would suggest. Reasons for this optimism are a 4- decade breeding effort which is overcoming very serious disease problems and producing higher quality, more versa tile cultivars with improved fresh, processed juice and wine characteristics; progressive, enthusiastic grape growers; a sympathetic state legislature and a long-term commitment by the Florida Grape Growers Association and the state, regional and national agricultural research establishment. The 1970 decade has resulted in progress with both bunch (Euvitis) and muscadine (V. rotundifolia) grapes in the areas of breeding, culture and processing. For the well-being of the grape industry, it is essential that these efforts be in creased during the 1980's and coupled with greater atten tion to fruit quality and fresh, processed and wine market ing. Grape development priorities of the 1980's will be dis cussed in terms of desirable, achievable goals for Of the 38 U. S. states which lay claim to a grape industry, Florida probably ranks about 19th in acreage (1). But there is promise of greatly increased future production. The decade of the 80's represents a very critical developmental 120 iflorida Agricultural Experiment Stations Journal Series No period. It is, therefore, instructive to describe where the industry came from and how it will develop and contribute to Florida agriculture over the next 10 years. Looking Back Except for the "Vineland" label which the Norsemen gave to Northeast North America about 1,000 years ago, the first recorded reference to grapes in the New World was from Spanish Florida in 1565 (17). From that time on inhabitants of Florida have been intrigued by the crop and consistently attempted to produce grapes and make wine (Table 1). These efforts have faced many problems over the centuries, and it is a tribute to horticulturists in the state that grapes are successfully grown. A few hundred acres of Vitis labrusca were reported in the early 1890's, but the vines lacked vitality and longevity and the effort failed (23). Grape acreage increased remarkably in the 1920's, and 4 to 5 thousand acres existed as of 1926 (19). Most plant ings were Munson hybrids 'Extra' (Florida Beacon), 'Car men', 'R. W. Munson' and 'Armalaga' and were devoted to fresh markets locally and in the Northeast. In the late 1920's disaster struck again in 3 formsdisease, depression and the fruit fly eradication program, disease being by far the most devastating. The Munson cultivars were plagued by short vine life (,~10 years); sus ceptibility to rot and diseases associated with a hot, humid climate; high cultivation and marketing costs; and weak markets. Vine degeneration, later identified as Pierce's disease, and the Mediterranean fruit fly eradication program effectively curtailed Florida plantings and resulted in a dramatic decrease in acreage, completely negating the com mercial potential of Florida grapes for several decades (21). Turning the Corner Fortunately, during and subsequent to the grape boom of the 1920's the seeds of future expansion were being Proc. Fla. State Hort. Soc. 93: 1980.