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THE AGRICULTURAL NEWS. 



Jam 



191S. 



PLANT DISEASES. 



PLANT DISEASES IN UGANDA. 



The Annuid liepi'rt t<'r 1916-17 of tlu' department of 

 Agriculturt- of the Uganda I'rotectorate contains references 

 to diseases of cultivated plants ■which are of interest for 

 comparison with those of oldei established cultivations else- 

 where in the tropics. 



The principal export crop is cotton, which it is estimated 

 covered 130,000 asre.s. Thi.s is followed in importance by 

 •coffee and rubber, and as small but developing industries, 

 by cacao, tea, sugar-cane, and tobacco. With regard to cotton 

 it is reported that as the result of adverse weather conditions 

 the losses were e.xceedingly heavy, and there was a large 

 percentage of stained and immature fibre. At the Govern- 

 ment Plantation heavy rain in the midst of the picking 

 season resulted in the proportion of bolls diseased and spoilt 

 by the weather reaching 90 per cent. Early and late cotton 

 was alike affected. Specific information as to the modes of 

 loss is scanty, but the Government Botanist, Mr. W. Small, 

 reports that anthracnose and areolate mildew were common. 

 Ko reference is made to the presence of Bacterium malni- 

 ctaruvi, the most widely distributed cause of serious disease 

 in this crop in the West Indies. 



The principal disease of coflFee appears to be die-back, 

 of uncertain causation, but following in many ca.ses on the 

 bearing of a very heavy crop. 



In this affection the primaries and twigs are most 

 frt'iui-ntly attacked at the joint, but the middles of young 

 primaries also often become affected. The affected part 

 turns black, and the tissue breaks down. In the early stages 

 such primaries present a sickly appearance, the leaves at the 

 ajiex bang down, wither, and fall off. The primaries eventu- 

 ally become dry sticks. In some cases laterals grow out 

 from one or two of the older nodes. 



The fungus ColUtdtric/inin cofwnuin, Noack, is closely 

 associated with the disease, occurring even in its early stages. 

 It is not invariably present, and the Government Botanist 

 takes the view that it cannot be held responsible for the 

 die-back that occurs on such a large scale. 



It is the opinion of that officer that die-back, in the 

 sense of the weakening and frequent deaths of trees which 

 follow over-bearing, especially at the time of the first heavy 

 cro[i. is more to be feared in Uganda than the notorious 

 Hernileia disease. The latter is always present on Cofta 

 ara''U" but its effects have not the disastrous nature of 

 tho£e attributed to it elsewhere. 



Cerro.'pora oijf'eicola, causing a leaf and berry blotch, 

 is always to be found on coffee estates but was not much m 

 evidence during the year. A root disease characterized by 

 black rhizomor[ihs has occurred on coffee, cacao, tea, rubber, 

 and indigenous trees. 



The most harmful of Uganda cacao diseases is the die-back 

 familiar in the West Indian and other cacao districts, 

 .'— ificiated with l.aii<i<Uplidiii thenhrninat . A soft rot of the 

 pods i.s attributed to the .saini- fungus. A fungus identified 

 ;t.- C<ilttt"tri'ku)ii the'i'iri'iiiir /uiii. De\., causes a destructive 

 1 -ifdeoing of the pods. A similar diEease, possibly identical, 

 f'.curs occasionally in the We>t Indies, and has been recently 

 ; ii with causing reddi.'h Irowii hardened patches on 

 immature' p- dt in St Vincent. 



(>n Heven. die-back attributed to the same fungus us in 

 '.\s been fAi:'\ common. Other recognized 

 have not ,- ytt made llieir appearance in 

 I..- '■ ,a:..'. i ' i.^aiions. 



The conclusion reached is that Uganda is srill in> 

 a favourable position as regards the frequency of plant 

 diseases and the haim done by them, but it is recognized 

 that this state of affairs cannot be expected to continue for an 

 indefinite period. 



The reference to root diseases of permanent crops indicates 

 the approach of a troublesome stage which has to be passed 

 through in all tropical countries where plantations are 

 established on foiest land. The fungi encountered vary with 

 the situation, but have this character in common, that they 

 spread from stunqis and logs and soil rich in the accumulations 

 of forest debris. It their advent is foreseen, much can be 

 done to diminish their effects; if early precautions are 

 neglected, depressing losses have to be faced, and the problem 

 of eradication becomes far more difficult. 



W. X. 



FACTORS AFFECTING THE SMUT 



DISEASE OF CORN 



In view of the occurrence — varying in intensity, but never 

 as yet reported to be severe — of the smut disease of Indian 

 corn in these islands, the following summary of conclusions 

 reached in a study of the fungus by E. J. Piemeisel (Pliyto- 

 ■pathology, Nil, I, 294-307) is reproduced:-- 



1. The infection of corn by i'slilago Ztae (Beckm.) 

 Unger., is purely local; no evidence of .systemic infection was 

 obtained. 



2. When very young plants become infected they are 

 often killed. 



3. Injury to the host plant, close planting, very early or 

 very late planting, and growth on rich soil are conducive to 

 heavy smut attacks. 



4. Vigorously growing plants, between 2 and 3 feet 

 high, are most susceptible to smut attack. 



5. The spores of V. Zeae can cause infection either 

 when young or old. Spores germinate readily as soon as 

 mature, and retain their viability for several years; infection 

 was obtained by inoculating corn plants with spores five 

 years old. 



(i. The cornsiiiut fungus does not lose its virulence 

 i|uickly when grown on artificial media. 



7. The spores of U. Zent, almost without exception, 

 lost their viability after having been kept in a silo for a few 

 weeks. 



.s;. The factors causing spores to lose their viability in 

 the silo have not been determined definitely; it seems prob- 

 able that the silage acids, especially acetic, may be the 

 destructive agents. 



9. Sporidia wtie kept in pure culture continuously for 

 three and a half years, at the end of which they remained 

 viable. Inoculation experiments with the same material 

 gave inconclusive results. 



10. Sporidia were desiccated for about five months with- 

 out seriously impairini; their viability. 



1 1. Krcczing injures sporidia but litllo; alternate freezing 

 and thawing, however, is injurious to moist sporidia, less so 

 to desiccated sporidia. 



12. The oplinium temperature for the budding of 

 sporidia is between 20° and 2(> C, the maximum at about 



lO'L'., and the thermal death point near 46°l'. 



13 S|poridia can germinate and bud in silage juice 

 but are injured in a .solution containing acids in the propor- 

 tionate concentraliMti in which they occur in silage 



14. The ability of sporidia, as Weil as spores, to with- 

 stand unfavourable cfmditions is very significmt in explaining 

 .some of the facts in the parasitism of U. Zeat- 



