Duffus, James E. : James E. Duffus is Research Plant Pathologist, U.S. Department of Agriculture, U.S. Agricultural Research Station, Salinas, California

Flock, Robert A. : Robert A. Flock is Entomologist, Imperial County Agriculture Department, El Centro, California.

Extremely large whitefly populations oc - Crop losses caused by curred during the summer and fall of 1981 , whitefly - transmitted inducing a series of viruslike disease epidem - ics on many important crops in California agents could change the and Arizona desert production areas . Popu - agriculture of the desert . lations , mainly of sweetpotato whitefly , Bernisia tabuci ( Genn . ) , exploded , attracting the attention of the national news media and causing losses amounting to a reported $ 100 million to growers and consumers . x 0 Cotton yields were down significantlydur - 7m ing the 1981 growing season and were also Bernisia tabaci adversely affected by reduced grade due to stickyhoneydew accumulations on the bolls . Damage worth $ 8 million to cantaloupe , wa - termelon , and other melons and squash in the WhitefIy = transmitted late summer and fall of 1981 resulted in ap - proval of a disaster loan program by the Farmers Home Administration . Lettuce disease complex of plantings were virtually 100 percent affected with severe yellowing symptoms ; yield losses the Desert Southwest estimatedat 50 to 75 percent , when combined with a lower acreage planted , resulted in very low production and extremely high prices . James E . Duffus 0 Robert A . Flock Losses of sugarbeets , infected early in their growth cycle , were estimated at 20 to 30 per - cent . The sweetpotato whitefly has occurred in California since the late 1920s but had never before attained such high populations and widespread distribution in the Southwest . Disease agents transmitted by whiteflies are encountered mostly in tropical areas but occur also under subtropical and even tem - perate conditions , depending mainly on the species that serves as their vectors . Bemisia tabuci is the most important of the whitefly vector species and , although not as important as aphids on a worldwide basis , is responsible for the natural spread of economically im - portant diseases in a band around the world between the 30 " parallels south and north of the equator . Some 30 or more diseases have been re - ported to be induced by the feeding of infec - tious whiteflies , including cotton leaf curl in Lettuce infectious yellows on greenhouse ' Bibb ' lettuce , spread Sudan and Nigeria , tobacco leaf curl in Java , by the whitefly , can be confused with aphid - transmitted viruses . India , and Africa , cassava mosaic in Africa , by rolling , yellowing , reddening , vein - tural Research Station indicate that the and legume diseases in the Western Hemis - clearing , and brittleness of affected leaves . whitefly disease complex in the desert agri - phere . It is probable that in many cases the diseases are caused by the same entity or by cultural area is caused by at least three major An early objective in studying the whitefly - agents . It should be noted , however , that variants adapted to specific host plants . transmitted disease complex in the desert was Disease agents transmitted by whitefliesin - field isolations from wild and cultivated to determine how many agents were respon - plants have thus far concentrated on rela - duce three major types of disease . One group sible for the crop losses , how the agents were tively few hosts , and still other whitefly - induces color deviations of the variegation transmitted , and what were the common crop transmitted diseases may well play a role in type on affected leaves . Another causes leaf - and weed hosts of the pathogens . With this the disease syndrome of desert plants . Each curl - type symptoms ; mosaic symptoms are information , the individual disease systems of the three major groups of whitefly - not evident , but infected plants show crink - could be more systematically studied and transmitted diseases is represented . The first , ling , vein etching , and leaf enations , or control attempted through vector , host , or growths . The third group includes typical cotton leaf crumple , first observed in 1948by pathogen management . Insect transmission studies conducted in Dickson and Laird in the Coachella and Im - yellowing - type diseases , characterized by our Salinas laboratory at the perial valleys , is stunting of infected plants and accompanied U.S . Agricul - of the leaf curl type . Leaves 4 CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982 severely reduced commercial lettuce yields in desert areas during Beds of â?? Salinasâ??lettuce in a field of â?? El Toroâ??are virtually 100 per - cent infected with lettuce infectious yellows virus . The disease the 1981 - 82growing season . Squash leaf curl is characteristic of mosaic - type diseases . It in desert field cotton in 1981 Cotton leaf crumple was widespread severely damages squash , pumpkin , watermelon , and beans . and affected nearly all of the crop . stunting of affected plants is characteristicof disease was first reported in of infected cotton plants are puckered and 1981 by Flock this virus on a wide range of commercial and appear warty on the upper leaf surface . and Mayhew but was recognized as early as weed hosts . Symptoms on most hosts are al - Flower petals show similar puckering , espe - 1977 . Although termed â?? leaf curl , â?쳌 the most identical to those caused by aphid - diseaseis more characteristicof the infectious cially on the margins . Young greenhouseand transmitted viruses of the yellows complex be markedly stunted . field - grown plants may variegation or mosaic type of syndrome . and could readily be confused with them . The disease was widespread in desert cotton Margins of infected squash leaves are curled This disease severely reduced lettuce yields in 1981 and affected nearly all of the crop . A and the veins thickened and distorted . Inter - ( 50 to 75 percent ) in desert areas during the crumple disease of bean , with distortion of veinal tissue becomes chlorotic or mottled 1981 - 82 growing season and was responsible the pods , has been observed in greenhouse with pronounced vein banding . The disease for large losses in sugarbeet production in the of the virus on cotton and plants . Effects causes severe damage on squash , pumpkin , same areas . bean yield reductions have not been deter - watermelon , and beans . The third distinct disease is induced by the of our early host - range labora - Analysis mined . newly described lettuce infectious yellows The second disease , widespread in desert of the important crop , tory data on some or reddening and virus . Interveinal yellowing production weed , and ornamental plants grown in the areas , is squash leaf curl . This CALIFORNIA AGRICULTURE , NOVEMBER - DECEMBER 1982 5 Host range of the major whitefly - transmitted diseases of the southwestern desert Lettuce Cotton Squash Lettuce Cotton Squash infectious leaf leaf infectious leaf leaf curl Hosts yellows crumple curl Hosts yellows crumple Crops Weeds and ornamentals - - - Alfalfa Anoda + * - - Bean - + * + ' Beta ( wild beet ) + ' - - - - Capsella ( shepherd's purse ) + * - - Cantaloupe + ' - - - Chenopodium ( goosefoot ) + ' - - + * Carrot Convolvolus ( morning - glory ) + ' - - - + Cotton + ' - - Cucumber + ' - - Dichondra + ' - - Malva ( cheeseweed ) + * + ' - Lettuce - - Melon + * Physalis ( ground cherry ) + ' - - - - - Okra Rumex ( dock ) + ' - - Safflower + ' - - Sonchus ( sowthistle ) + * - - Squash + ' - + Tagetes ( marigold ) + * - - - - Sugarbeet + ' Taraxacum ( dandelion ) + * - - - - Sunflower + * Trifolium ( clover ) + * - - - - - Tomato Zinnia + ' - - Watermelon + ' - + ' * New hosts not previously reported . + = Positive recovery from inoculated plants . - = No recovery from inoculated plants . I Jan I Feb I Mar I Apr 1 May1 Jun I Jut 1 Aug I Sep I Oct INov 1 Decl Growth period of crops susceptible to lettuce infectious yellows virus . desert ( see table ) givesbasic information that of squash leaf curl . control in crops and along roadsides , ditches , clearly distinguishes the diseases , clears up The lettuceinfectious yellows virus appears and the like . some misconceptions , and suggests possible to affect a wide range of crop and weed plants With the limited biological data on white - control measures . ( see table and figure ) . It would be difficult at fly populations and basic information on this stage of the research to design a simple reasons for seasonal variations , it is impos - Early observations by field representatives and growers suggested that cotton played an crop management strategy that would elim - sible to predict the economic impact of the important role in the disease cycle on lettuce inate or greatly reduce the incidence of this disease complex on future crops . We do so , our evidencein - and other crops . If this is virus in wild species . However , a slight altera - know , however , that reservoirs of whitefly - dicates that cotton serves only as a source of tion in the growth cycles ( planting and har - transmitted agents are widespread in south - whiteflies but not of viruses for the major vesting dates ) of a few crops might affect the western desert plants and that losses caused crops except beans . The cotton crumple - leaf occurrence of two viruses . A two - to three - by these agents could change the agriculture disease agent seems to have a very limited of cantaloupe , other week period free of the desert . Much additional host and bio - host range , including cotton , bean , and melons , and squash in July or August would logical information is needed to fully charac - Malva ( cheeseweed ) . Elimination of off - greatly reduce the incidence of lettuce infec - terize the disease - inducing entities now season ( stub ) cotton and reduction of cheese - tious yellows in the area and would probably known in the complex , and the possible role weed would probably control this disease . greatly reduce squash leaf curl . of additional diseases must be determined . General control methods should be used The squash leaf curl virus also has a limited until a better understanding of the infectious host range : in studies thus far it has been James E . Duffus is Research Plant Pathologist , yellows virus , the diseases , and possible resis - recovered only from squash , watermelon , and US . Department of Agriculture , rUS.Agriculturalt tance can be studied . General strategies our labora - bean . The agent under study in Research Station , Salinas , California , and Robe include reduction of whitefly populations , tory does not infect melons , cotton , lettuce , A . Flock is Entomologist , Imperial County Agri - culture Department , El Centro , California . The isolation of plantings from large acreages of or sugarbeets . A squash - , watermelon - , and of authorsgratefully acknowledgethe cooperation bean - free period in desert growing areas infected plants , destruction of infected plant - James M . Thompson , Wilber - Ellis Co . , Glendale , Arizona . ings immediately after harvest , and weed would probably greatly reduce the incidence 6 CALIFORNIAAGRICULTURE , NOVEMBER - DECEMBER 1982