Calavan, Edmond C. : Emeritus, Department of Plant Pathology

Eckert, Joseph W

Johnson, Edward L. V. : Department of Plant Pathology

Van Gundy, Seymour

of the diseasewas demonstratedand preventive Diseasesand their control measures were developed by CES scientistsin the 1930s . Clean planting stock was found essentialto Edmond C . Calavan , Professor , Emeritus , prevention . Departmentof Plant Pathology ; Seymour D . Van Gundy , Professor , Tristeza , exocortis , and stubborn Department of Nematology ; A mysteriousmalady called â?? quick decline â?쳌 Joseph W . Eckert , Professor , began killing orange trees in southern Californiain Department of Plant Pathology ; and 1939 . Within 25 years it caused the loss of three Edward L . V . Johnson , Staff ResearchAssociate , Department of Plant Pathology million trees , mostly sweet orange on gummosis - resistant sour orange rootstock . Some trees wilted quickly , others declined slowly and lost foliage . everal times in the past 75 years , Californiaâ??s CES began a crash research program in the early citrus industry faced catastrophiclosses from 1940s . J.M . Wallace and H.S . Fawcett showed that S In every case , timely fruit and tree diseases . viruscaused the disease , and R.C . Dickson tristeza research greatly reduced the damage . and colleaguesfound that it was transmitted in California by the melon aphid . Further research Gummosis and psorosis , old maladies determined that Troyer and Carrizocitrangeswere Heavy losses of trees from gummosis ( foot rot ) in tolerant to tristeza , so these rootstockswere used 1900 - 15 alarmed citrus growers and stimulated extensively for replants and new orchards . their interest in research , thus providing impetus a time at least , brought under Tristeza was , for to the new CitrusExperiment Station ( CES ) . substantialcontrol . Recent spread of new strains Gummosis - affectedtrees turn yellow , decline , and of tristeza virus that cause severe damage to sweet die if the trunk or primary roots are girdled . The orange and grapefruit trees on all rootstockshas 1913 , to the appoint - gummosis outbreaksled , in stimulated additionalstudies at the Citrus Re - ment of a plant pathologist , H.S . Fawcett , to the search Center to contain the new threat . CES staff . Dr . Fawcett determined that phytoph - Citrangeand trifoliate orange rootstocksare , un - thora fungi were killing citrus bark near the soil fortunately , susceptibleto exocortis , shown by gummosis disease level and were causing both the pathologists in the experiment station to be caused and brown rot of fruits , which were especially a viroid , which is much smaller than any virus . by destructive in lemon orchards . Dr . Fawcett and Dr . Exocortisviroid causes stuntingand bark scaling L.J . Klotz developed practical phytophthoracon - of these two rootstocks . It is spread by tools and mix - trol measures with applicationsof Bordeaux through grafting , but can be largely prevented by ture and surgical excision of lesions . the use of clean stock , sanitation , and resistant Dr . Klotz and colleaguessubsequentlyworked rootstocks , such as sweet orange , grapefruit , and extensivelyon the life cycles of these water rough lemon . molds , and refined brown rot control measures 10 years old , a strange When CES was less than using copper - bearingsprays . Sourorange root - stunting and fruit deforming disease , then called stockswere found gummosis - resistantand were sheepnose , appeared on navel oranges in the Red - soon extensivelyused in California . Recently , lands area . Growers found it was perpetuated by plant pathologists have obtained good control of propagationand called it stubborn , because af - fruit infections of Phytophthma root , trunk , and fected trees would not respond to treatment . CES spp . by treatments with the systemic fungicides , scientistsand growersworked together to show its metalaxyl ( Ridomil ) and fosetyl - A1 ( Aliette ) . Nearly 50 years after its discovery , transmissibility . Gummosis is not the only potentially catas - U.S . Department of Agriculture several CES , trophic disease the Californiacitrus industry has ( USDA ) , and overseasscientistsdiscovered that faced in this century . Psorosis ( Californiascaly citrus stubborn disease is caused by a subbacterial bark ) , an incurablegraft - transmissibledisease Spiro - organism new to science , the mycoplasma destructiveto oranges , grapefruit , and tangerine plasma citri , and that the stubborn organismalso rare among the seedlingtrees of Cal - trees , though infects many ornamentals , vegetables , and weeds a century ago , iforniaâ??scitrus industry more than after transmission by beet leafhoppersand two became widespread and destructivein grafted speciesof Scaphytopius ( leafhoppers ) . Control is trees before 1900 . Apparently due to virus infec - by prevention and by removal of stubborn trees . tion , psorosis caused bark scalingand death of in - ner wood , followed by gradual decline of affected The clean stock program trees . It remained the most importantgraft - The need for disease - freepropagative budwood transmissiblecitrus diseasein California until the was evident long ago , but efforts to find such 1940s , when it was overshadowed by tristeza . material were thwarted for many years by numer - of lesioned areas delayed or Scrapingthe bark ous symptomlesscarrier trees and the lack of tech - reduced tree damage by psorosis . Transmissibility niques for eliminatingdisease organisms from otherwise desirabletrees . The first effective move toward a clean stock program for citrus was taken by the CES and the 1930s , CaliforniaDepartment of Agriculturein the m n after psorosis was found graft - transmissible . A registration program was established to identify parent trees from which nurseries could obtain CR - 10 budwood free of the psorosis pathogen . This pro - to turgid citrus fruits . Recently , experiment station researchers found that metalaxyl ( Ridomil ) erad - gram prospered until tristeza virus overran most as well as hot water icates phytophthora infections registered trees in the 1940s . With losses from virus and other bud - perpetuated diseases mount - without the iqjury potential . ing in the 1950s , the citrus industry sought help Alternaria stem - end rot is a major factor limit - CES in establishinga â?? Variety Improvement from ing the storage period of lemons . Bartholomew Program â?쳌 to provide disease - free , true - to - name Altmnaria infected the stem showed in 1926that budwood for increase by nurseries . The psorosis buttons of the fruit early in their development on registration program was reactivated quickly , but the tree and , at harvest , the fungus was deeply it took about 10 years for the experiment station imbedded in the tissue , where it survived post - to produce trees that indexed free of all known harvest fungicidaltreatment . Many buttons were diseases . infested , but the fungus attacked the fruit only Despite some problems with stubborn disease , after the buttons became senescent . W.S . Stewart expanded citrus registration and certification , now demonstrated in the early 1950sthat postharvest called the â?? Citrus Clonal Protection Program , â?쳌 treatment of fruit with 2,4 - D delayed senescence has , thanks to strong support by the industry and and postponed the attack . Today , all lemons pro - excellent cooperation by the California Depart - duced in Californiaare treated with 2,4 - D in the ment of Food and Agriculture , Cooperative Exten - packinghouse to control alternaria stem - end rot during storage . sion , and USDA , provided a good supply of clean budwood for increase by nurseries during the past Work continues at the Citrus Research Center to 13years . This goal was achieved only by develop - reduce losses caused by blue and green molds , the ing methods , or adapting those developed else - most serious market diseases of citrus fruits pro - where , for detection by indexing and elimination duced in California.These investigations have two of all pathogens present in desirable clones . The purposes - to prevent infection of injuries on the ongoing program at the Citrus Research Center is a Penicillium and to inhibit sporulation on fruit by joint effort of the departments of Botany and diseased fruit , thereby preventing spoilage of Plant Sciencesand of Plant Pathology to discover sound fruit . Research was started in the 1950sto ways of eliminatingand avoidingPhytophthora find a substitute for biphenyl , a postharvest fungi - and other fungi , psorosis , tristeza , stubborn dis - cide then severely criticized in European markets . in ease , vein enation - woody gall , exocortis , cachexia , See - butylamine ( 2 - aminobutane ) , discovered this program and released to the industry in the ringspot , concave gum , infectious variegation , early 1960s , has been widely used to prevent crinkly leaf , tatter leaf - citrange stunt , and others . Thenno - therapy and shoot - tip grafting have been infection of lemons during storage and of oranges effective in eliminatingpathogens . during ethylene degreening . During the past 15 years , several new fungicides have been investi - Thus far , the clean trees have been successfully are es - gated , each more effective than its predecessor in maintained and the more important ones tablished in the Lindcove Foundation Block east of preventing infection and inhibiting sporulation of Penicillium . Visalia . The Californiaprogram has reduced citrus as a model The intensive use of see - butylamine , thiabenda - tree disease losses and has been used A compre - Penicil - for similar projects in other countries . zole , and benomyl has selected strains of lium resistant to these fungicides , resulting in hensive clean stock program is essential to the their failure to control decay . Packinghouse man - continued production of healthy citrus trees . agement strategies for controlling the resistance Postharvest decay , a costly mess Two new problem are being actively investigated . fungicides , imazalil and etaconazole , not yet avail - Decay of citrus fruits after harvest has caused able for commercial use , can control strains of substantial losses to growers and consumers for Penicillium resistant to fungicides now in use . more than a century . The Californiacitrus industry at Riverside have shown that Investigations estimated loss due to decay during 1979at $ 84 etaconazole also effectively controls sour rot , a million . The major postharvest diseases of citrus disease not controlled by any existing commercial ( Penicillium ) , fruits are blue and green molds treatment . sour rot ( Geotrichwm ) , brown rot ( Phytophthora ) , and alternaria stem - end rot . Brown rot and alter - Some fungi are good for citrus fruit on the tree ; naria rot begin from infections of Mycorrhizae are beneficial fungi that grow in blue and green molds and sour rot start from symbioticassociation with roots and aid in the up - spores deposited in iqjuries . take of many nutrients , particularly phosphorus . L.J . Klotz found that copper fungicides were in - as â?? biotic These fungi are sometimes described effective against brown rot if the fungus had fertilizers , â?쳌 because they can be partially substi - penetrated the peel , but submergingfruit with in - tuted for high fertilization . Unfortunately , concen - cipient infections in hot water ( 115 ° - 1180F ) for trations of methyl bromide used for fumigating two to four minutes killed the fungus . Various are also lethal modifications of this treatment have been used in citrus nurseries to kill soil pathogens to mycorrhizal fungi . Californiapackinghousesfor many years to control U.C . , Riverside , researchers collected , screened , brown rot , despite the potential for heat damage Glomus and selected the most efficient strain of fasciculatus , a mycorrhizal fungus associated with citrus . The relative dependencies of various citrus CR - 11 rootstocks on mycorrhizae in fumigated low - root development and growth of young citrus trees . Thomas also called attention to the danger nutrient soil were determined . Information from of distributing this nematode on nursery trees . in 26 soils was computer analyzed , pro - responses Some 30 years later , after the development of soil A ducing formulae for predicting citrus growth . fumigation , the California Department of Food commercial production system for mycorrhizalino - and Agriculture established a nursery certification culum was developed recently and has been program , which continues to ensure the produc - adopted by one California nursery . tion of clean planting stock . Although Thomasshowed the importance of Citrus nematode citrus nematode , no means were available for con - Since the citrus nematode , Tylenchulussemi - trolling it on bearing trees and little attention was penetrans Cobb ( slow decline ) , was first dis - focused on this citrus production problem until covered in Los Angeles County by J.R . Hodges in C . Bainesjoined the experiment station Richard 1912 , the CES has been the international center in 1947 . At about that time , the chemicals for research on its biology and control . Nathaniel 1,3 - dichloropene ( 1,3 - D ) and ethylene dibromide A . Cobb , who had just joined the Bureau of Plant ( EDB ) were discovered to be effective soil fumi - Industry in Washington , D.C . , immediately des - gants for the control of nematodes in pineapple . cribed this species and organized an international Baines seized the opportunity to study these and survey , which established that this pest was other chemicals for control of the citrus nematode . He used this example of worldwide in distribution . During the 1950sand the 1960sthe station con - a plant parasitic nematode to draw attention to ducted two major interdisciplinary programs to the scientific and economic importance of soil - find solutions to the â?? citrus replant â?쳌 and the inhabiting nematodes to agriculture and soon â?? â?? citrusgrove rejuvenation â?쳌 problems . The established the first Nematology Laboratory . research by Baines and his colleaguesin Nema - Results of a thorough study by Edward E . tology , Plant Pathology , Plant Sciencesand Soil Thomas of the CES , completed in 1923 , demon - Sciencesclearly demonstrated that these problems strated that the parasite was widespread on citrus could be solved by control of the citrus nematode rootlets in California and that it greatly retarded by soil fumigation and use of tolerant rootstocks . The citrus nematode is a semi - endoparasitic worm , feeding internally in citrus roots while a major portion of the body remains external to the root surface . Severely infected rootlets may con - as many as 5,000 females per gram . Each fe - tain male produces 100 to 200 eggs , which hatch into larvae that reinfect the spring root flush . Feeding by the nematodes reduces the number of new root tips , decreases uptake of nutrients and water , and increases the rootâ??s susceptibility to soil fungi . During the late 1940sand early 1950s , Baines showed that young nematode - free citrus trees replanted in heavily infested citrus orchard soil as much as 60 percent when com - were stunted pared with trees planted in fumigated soil . Fumi - A . Phytophthora gummo - gation of citrus soil before replanting is now sis , the worst fungus standard practice throughout the Californiacitrus disease of California areas . citrus trees . In the search for nematode resistance in citrus species and close relatives , CES scientists iden - 6 . Indexing for tristeza as highly resistant to tified the trifoliate orange virus . Disc from leaf of Phytophthora . They the citrus nematode and to candidate tree is placed tested and released the Troyer citrange , now a in Mexican lime , which widely used rootstock in California and other will develop symptoms if candidate is infected . citrus growing regions . In about 1950 Baines also pioneered in the de - C . The first certified velopment and use of the soil fumigant , citrus nursery tree pro - 1,2 - dibromo - 3 - chloropropan ( eDBCP ) on produc - duced in California and ing citrus . DBCP was the first nematicide that its developers in 1970 : could be used around living plants , and it im - D.A . Newcomb , S.M . proved growth and production of citrus by 10 to Mather , W . Reuther , E.C . 32 percent . Unfortunately , use of DBCP was can - Calavan , H.W . Duncan . celled in 1977 , a loss that caused major concern to the citrus industry and a renewed research effort D . Small , misshapen fruits caused by stub - at the University for new chemical and biological born disease , compared alternatives . with normal orange . E . Spiroplasrna citri , cause of citrus stubborn disease . CR - 12