Jones, Winston W. : Emeritus, Department of Botany and Plant Sciences

Reuther, Walter : Emeritus

yield , and distinctive leaf symptoms ( small , nar - CuIturaI practices row , yellowed leaves ) . University of California sci - entists played a key role in demonstrating that the Walter Reuther , Professor of Horticulture , disorder was caused by a deficiency of zinc , a and Horticulturist , Emeritus , nutrient essential in minute amounts for normal Winston W . Jones , Professor of Horticulture growth . They showed that it could be effectively and Horticulturist , Emeritus , and economically controlled by periodicallyspray - Departmentof Botany and Plant Sciences ing the foliage with dilute solutions of zinc salts . This micronutrient deficiency was prevalent in citrus regions of the world , and today zinc sprays ince the Citrus Experiment Station began in are an integral part of the cultural program in 1907 , a major concern has been development S most well - managed orchards . of more efficient fertilizer practices . Early Research at the CES also contributed important experiments by CES scientists , primarily in the or - information on causes and control of other micro - chards of cooperating growers , showed that on as manganese , copper , nutrient deficiencies , such most , but not all , California soils , citrus trees do iron , and boron . Studies showed that heavy fertili - not respond to potassium and phosphorus appli - zation with potassium salts eventually induces a cation . Substantial amounts of nitrogen , however , magnesium deficiency in California citrus or - have been required annually to maintain good as in other citrus regions . Indeed , results chards , vigor and yield . Later ( after World War I ) , it was obtained in California and elsewhere clearly dem - found that animal and green manures could be re - onstrate strong cumulative reciprocal relations as nitrogen sources by chemical sources , placed among applied nutrient elements , and show that which , among other advantages , were more effi - persistent use of unneeded fertilizers almost ciently used by the tree and became progressively always leads to undesirable side effects over a cheaper per unit of nitrogen . period of years . In the last 30 years , the development of auto - As a result of CES studies , the effects of mineral mated pressure irrigation systems ( sprinkler , drip , nutrition on fruit quality are better understood . micro - jet , and the like ) have made it possible to Extensive orchard and solution culture experi - plant orchards on virgin hillside soils . Such soils ments isolated the effects of deficiencies and ex - are generallylower in native fertility than those in cesses of both macro - and micro - nutrients on juice most older orchards planted on relatively flat composition , fruit size , and juice percentage , as valley floors and irrigated by traditional gravity well as on peel color , texture , thickness , and sus - systems ( such as basin and furrow ) . ceptibility to certain rind disorders . For example , a Research has shown that many hillside orchards potassium deficiency is associated with small fruit require potassium fertilization . Such soils tend size ; a high juice percentage low in acid ; thin , to have a lower inherent available potassium con - smooth , well - colored peel ; and a tendency toward of tent and also lack the accumulated residue excessiverind splitting . Very high levels of potas - potassium from repeated fertilization with the sium are associated with large fruit having thick , potassium - richorganic manures that were com - Zoarse - textured , poorly colored peel , and a low monly applied to early California orchards on pice content high in acidity . These effects of high valley floor soils . potassium are accentuated by high nitrogen levels . Also , it was found that fertilizers , especially Analysis of leaves to diagnose deficiencies and nitrogen sources , might have important side ef - 3xcesseshas become a primary guide in the devel - fectson tilth of many soils , which could depress or 3pment of efficient fertilizer practices in many im - as a even override the yield benefits of nitrogen of the world . It has been portant citrus regions nutrient . For example , repeated applications of specially valuable in avoiding the excessive use of sodium salts of nitrogen have a cumulative effect inneeded fertilizers in orchards . Also , use of leaf on water penetration , which gradually depresses malysis in research has greatly reduced the need yields because of soil moisture deficits not easily â?? or large numbers of expensive , ponderous or - remedied by revising irrigation practices . : hard experiments . Again , CES scientists played a One widespread problem that led to the estab - < eyrole in developing this valuable technique . lishment of the CES was the disorder called â?? little Possiblepollution of underground water with leaf , â?쳌 which was associated with low vigor , poor : hemicals used in fertilization , especially nitrates , ias become of concern in the last decade with re - prd to human health . CES research has shown . hat such pollution can be avoided or greatly . educed by leaf analysis combined with leaf appli - : ation of a part of the required nitrogen in the brm of urea . As a result of the efforts of CES scientists and CR - 7 others , few well - managed citrus orchards in Cal - when applied in minute concentrations , could pro - ifornia suffer yield restraints from faulty fertiliza - foundly influence growth and other processes in 11 , most orchards tion , whereas before World War plants . CES played a significant role in applying produced below potential because of poor fertil - this basic knowledge to practical citrus culture . izer practices . An early 1950sapplication was the development of a technology for using preharvest sprays of Weed control 2,4 - dichlorophenoxyacetiaccid ( 2,4 - D ) and related compomds to delay and reduce abcission ( drop ) of After World War I , heavy , tractor - drawn equip - mature fruit of most citrus varieties grown in Cali - ment for orchard cultural operations became fornia . Another important effect of such , â?? stop 1930s , CES investi - increasinglycommon . By the drop â?쳌 spray treatment is the delay of degenerative gators observed that in most orchards the frequent changes that accompany abcission and reduce traffic of such heavy equipment caused a gradual fresh market or processingquality of fruit . In ad - deterioration in soil structure . Frequent mechan - dition , 2,4 - D is used to some extent to improve ical cultivation for weed control was especially fruit size of Valencia oranges and grapefruit by harmful . The result was reduced water penetra - dilute sprays applied to very young fruitlets . Also , tion , impaired root development , and related ef - 2,4 - D is sometimes added to oil pesticide sprays to fects that reduced tree vigor and productivity . lessen leaf drop caused by the oil . An early disorder of In the 1940s , experimentation began with light citrus called â?? little leaf â?쳌 Another growth regulator , gibberellic acid ( GA , ) , as an alternative petroleum oils for weed control was corrected after U.C . now commonly used in many citrus growing 11 , for mechanical cultivation . After World War scientists traced it to a regions , was pioneered in the 1960sby CES sci - CES started intensified research on the use of oils deficiency of zinc . entists . In California , preharvest GA , treatments as herbi - and other chemicals being developed are now widely used to delay rind degeneration of cides . These investigations clearly showed that a navel oranges . By greatly reducing such disorders â?? clean culture â?쳌 or â?? nontillage â?쳌 system of soil as rind staining , water spot , and sticky rind afflic - management using chemical herbicides would be ting navel oranges held on the tree past the mid - practical in most California citrus orchards . In ad - point of physiologicalmaturity , GASsignificantly dition , they demonstrated its advantages over lengthens the effective harvest season , improving mechanical weed control in cost and improved soil returns to growers and quality of fruit to con - tilth , as well as in reduced water use , frost hazard , sumers . GA , is also used to some extent , especially and soil erosion . in coastal regions of California , to delay fruit By the mid - l960s , almost all Californiacitrus maturity of lemons . Late fall applications of GA , growers had converted from mechanical to chemi - shift the major flowering period so that more cal weed control - the first to adopt the nontillage fruits mature during the summer when demand is system of soil management on a wide scale . This at its peak . Research in Californiawith GA , led to system has since spread to many other orchard its use in Florida to improve fruit set of the Or - crops around the world , and even to such annual lando tangelo ( a mandarin - grapefruit hybrid not as corn , cotton , and many others . row crops Effects of fertilization on grown commercially in California ) , and to delay quality of Valencia rind degeneration of seedless grapefruit varieties , Plant growth regulators oranges on phosphorus - a problem in Floridaâ??sclimate . By the end of World War 11 , plant scientists in deficient soil . From left : The naturally occurring ( endogenous ) growth laboratories all over the world had shown that cer - no fertilizer , nitrogen regulators have been little studied , but the work alone , phosphorus alone , tain organic chemicals called growth regulators , that has been done indicates they are related to nitrogen and phosphorus . fruiting behavior . This is a promising field for future work . Effects of climate Growers early - on observed striking differences in season of maturity and quality of fruit produced in the several Californiaregionssuitable for citrus culture . For example , Valencia oranges mature about 11 months after flowering in the hot desert climate of the CoachellaValley , and may be stored ac - an the tree about three months after reaching ceptable harvest maturity . In the cool , more humid of the Oxnard Plain , 13 to 14 zoastal climate months are required to reach harvest maturity , and the fruit may be stored on the tree another five or six months before unacceptable deteriora - tion and loss occur . Many growers and marketing people attributed such differences to climatic liversity rather than to differences in soilsor was xltural practices . But definitive information ! acking , and there was much controversy . II confirmed that Studies at CES after World War climatic diversity was indeed the major cause of cR - a the differences observed . The shape of the cumu - lative seasonal net radiation curve is more closely related to the form of fruit and rate of maturation than are other more conventional indices of sea - sonal heat . Net radiation is a closer reflection of the ambient thermal energy level ( temperature ) of plant tissues than is ambient air temperature . Cor - relative studies in orchards in various climatic regions and with fruiting trees in controlled tem - perature greenhouses indicated that differences in the ambient energy level during various stages of fruit development were the primary cause of the observed differences , and thus could not be appre - ciably modified by orchard management . Other studies have shown that heat stress in the hottest desert regions of California and the South - west can be a significant yield restraint . Unsea - Effect of climate on quality of Redblush grapefruit . sonably hot weather during the fruit - setting period All were harvested during last week in January from may cause heavy crop reduction , especially in genetically identical trees of the same age receiving navel and satsuma varieties . Heat damage to similar cultural treatment . Two fruits on left are from air temperatures exceed 104O F ( 40O leaves when two cool coastal valley locations ; two on right from hot C ) may impair photosynthetic capacity , cause leaf to very hot , arid interior valley climates . High temper - drop , and reduce yields . The number of days per atures produce large , early - maturing fruit and stimulate air temperatures exceed 104O F is a season when the pink pigmentation . measure of the heat damage potential to citrus fol - iage , but windiness may be an aggravating factor . Other cultural practices Planting distances : There are no fixed planting distances ; they vary with scion variety and root - stock . Considerable study by CES scientists and 25 - by % - foot others indicates a 20 - by 20 - foot to spacing may be about right for a mature orchard . Closer spacing produces more fruit per acre until the trees begin to crowd , after which shading out occurs , and the trees must be thinned to maintain yields . Hedgerow planting is sometimes used , but there are disadvantages . Pruning : An important finding of research on as pruning has been that citrus does not respond do other fruit trees , such as apple or peach . Prun - ing is not required to maintain fruit yield . Usually , spacing is not sufficient , however , and pruning is Typical nontillage Cali - required to keep the trees relatively small , permit - fornia citrus orchard , where herbicides replaced ting more efficient spray application and cheaper mechanical cultivation fruit harvest . This is especially true for lemons . by the mid 1960s . Because of labor costs , most pruning is now done by mechanical toppers and hedgers . that alternate bearing in Valencia can be reversed Fruit thinning : The number of fruit per tree and by the removal of all young fruit in July of the fruit size are usually inversely correlated . Because â?? on â?쳌 year . small fruit size is sometimes a problem with the Alternate bearing and time of harvest : Since Valencia orange and some mandarins , considerable CES was established , its scientists have studied , work has been done on the effects of thinning . Re - but not yet solved , the problem of alternate bear - sults have not been consistent ; hence , thinning has ing , especiallyin late - harvested Valencia orange . not become a common practice . Hand - thinning is Early harvest can reduce the intensity of alternate as yet , there is no chemical thinner expensive and bearing but is not practical because of marketing that will do a consistentjob . It has been shown problems . Recent research indicates that in some cases the depletion of carbohydrates results in the â?? off â?쳌 year . In other situations , the naturally oc - to be involved . curring growth regulators appear Work along these lines may produce an answer to the problem . CR - 9