€2 



THE AGRICULTURAL NEWS, 



Fei-.f-i AEV -23, 1918. 



PLANT DISEASES, 



NOTES FROM RECENT LITERATURE ON 



CITRUS DISEASES. - 



CITRUS SCAB IX KOETO i:in.'. •'. A. Stevenson {Bulletin 17, 



Department "f Ap-iadture and L'i''0"r). 



Among the various diseases of grapefruit in I'orto Kico, 

 citrus scab has beyond much doubt assumed a position of 

 first rank It is the chief agent in sending fruit to the cull 

 pile and in the lowering of grades. The disease has been 

 especially virulent during ihe past four or five seasons, and 

 in some cases has disfigured and rendered worthless the 

 larger part of the crop- 

 Citrus scab is an introduced disease which has probably 

 come by way of Florida, and been present many years. It 

 first app. -.red on sour orange and lemon, and remained 

 practically confined to these two hosts, appearing only 

 occasionally on young grape fruit trees until the excessively 

 wet season of 1911 12. In that period the grapefruit was 

 attacked suddenly and virulently in the BAyamon section, and 

 the disease has since progressed without cessation, invading 

 in turn most of ihe other citrus districts. 



On sour orange and rougli lemon tie disease is so 

 prevalent that it is quite usual for all the seedlings in nursery 

 beds to be badly distorted and stunted; other varieties of 

 lemon are repoited to have been equally subject to the 

 disease. 



The lime and the mandarin have not been noted as 

 susceptible to attack: the sweet orange is usually considered 

 as immune, though a few cases have been seen of trees 

 exposed • infection bearing scabby fruit, and the author 

 considers it not at all unlikely that this species may lose 

 its immunity at any time, as happeced to the grape-fruit. 



The Tiiumph variety of the- grape fruit is al^o immune, 

 but its commercial defects prevent its adoption. 



The disease attacks the fruit, leaves, and young twigs, 

 producing first minute translucent areas, which are suc^ 

 cecded by the corky outgrowths characteristic of the disease. 

 ^)ften on the fruit these run together and cover .i large part 

 of the surface. ^ oung fruil are frequently much distorted, 

 and in some cases caused to drop. The affection is super 

 ficial only, and increast.s rather than diminishes the keeping 

 fpiality of the fruit. 



The cause of the disease is a specific fungus infection, 

 .spread mainly by rain and wind. Atmospheric conditions 

 during and shortly after the flowering period determine the 

 severity ol the attack: wet cloudy weather favouring its 

 development. 



I'nliku many plant disease.s, citruH ecab shows a decided 

 preference for healthy vigorous trees, ami actually avoids 

 unhealthy or neglected trees. 



The only known means of controlling scab is by spray- 

 ing with a fungii-ide. Hordeaui mixture gives much the 

 best results, hiut has ()roved in practice to have the great 

 disadvantage of also destroying the fungus parasites of .scale 

 insects, and so leading to sevtre infestation <>f the groves. 

 Lime sulphur mix'aire is much less efficient, but i.'- free from 

 this objection, and is recommended for general use. in very- 

 bad ca.i^e- it may be necessary to alternate with ilordiau.x 

 iiiixtuie. 



So far as is known to the present writer, scab does not 

 occur in any virulent form in the Lefser Antilles. 



WOdli l;oT OK flTnCS TREES IX PORTll RICO, J. .\. 



Stevenson {Citndar P>). 



A serious amount of loss in Porto Rico arises from decay 

 of the wood of fruit trees, consequent on the inva.-ion of 

 stems and branches by wood-rotting fungi which gain 

 access by way of broken branches, pruning cuts, and the 

 lesions of various primary disea.ses. The author feels 

 assured that this type of trouble will play a most important 

 part in the decadence of citrus groves in the not very distant 

 future. Its ett'ects are often laid to other causes, and action 

 to prevent it is seldom taken. 



It is interesting to note the statement that as a general 

 rule the rebudding or topworking of citrus trees in Porto 

 Kico has not been successful. The poor results obtained from 

 •V system which works well in other citrus regions are 

 attributed to wood rot, following on sunburn of unprotected 

 branches and trunks, and on untreated or poorly treated 

 pruning wounds. 



The adoption of advanced methods of tree surgery is not 

 considered practicable under Porto Rican conditions. The 

 recommendations made are (I) to attempt to treat only simnle 

 cases, (2) to replace badly affected trees, and (3) to concentrate 

 on preventive measures. These consist in removing all dead 

 material, sterilizing exposed surfaces with corrosive 

 sublimate solution or Bordeaux paste, painting later with 

 gas tar (from the use of which no injurious ett'ects have been 

 noticed), and renewing the coating periodically until the 

 wound has healed. 



.\MX0R.\IAI, WATKl; REL.Vn0X.S IX CITRUS TUKEs, It. \V. 



Hodgson {UnSivnilii of CalirorriKi Piiltl'n-^itionii ni 

 A'iricultuval Sciencfs, A'ol. 3, No, L'). 



This and the publication referred to below re|jresent 

 attempts to secure by systematic experimental studies some 

 definite and accurate infoiniation as to the phj'siology of 

 the genus Citrus. Only by a thorough understanding of the 

 relations existing between normal trees and their environment 

 can one hope to find the underlying causes of the numerous 

 obscure troubles classed as 'physiological' diseases, to which 

 citrus trees in some districts are particularly prone. 



The present study, referring to trees grown in arid 

 districts in ('alifornia and Arizona, deals with a phase of the 

 excessive shedding of young oranges, known as 'June-drop,' 

 which has been experienced for years in those districts. 

 This annual loss of young fruit is most serious in regions 

 where rainfall and atmospheric humidity are lowest 

 lempeiature and solar radiation highest, and air movement 

 greatest during the growing season, .A hot wave in ,lune 

 1',I17, during which a temperature of lis K. was reached, 

 caused a drop so severe that practically the entire <'top was 

 lost in districts where shedding is not usually excessive. 

 Following a reference to various published observations 

 including those of W, I.. Halls on the cotton plant' 

 on the daily deficit of water in the tissue of plants exposed 

 to conditions promoting rapid transpiration, a deficit which 

 commonly leads to something like definite wilting in the 

 afternoon of each sunny or sunny and windy day, the author 

 remarks that the etiects cf such temporary deficit may be' 

 expected to occur to a'n' Unusual degree when citrus plants, 

 with a leaf structure adapted to somewhat hutntd troijical' 

 conditions, aregrown in. the dry interior valleys of .'California.' 

 With I Ik -se considerations in mind it was thouglu that the 

 shedding might bedueto ihe-e irregularities in n.iter relations 

 resulting in absci.'-sion in sf"> e way analogou> to the shedding 

 of cc'tton bolls. 



