liiO 



THE AGRICULTUKAL NEWS. 



June 15, 1918. 



PLANT DISEASES. 



ALGAL DISEASE (RED RUST) OF CACAO. 



In the annual return regarding the prevalence of pests 

 and diseases in .St. Lucia in 1917, Mr. A. J. Brooks, Agricul- 

 tural Superintendent, reports that algal disease of cacao 

 occurs on most estates at certain periods. Although speci- 

 mens were collected as early as 1902 by Mr. G. Whitfield 

 Smith in Grenada, the disease has previously attracted 

 little or no attention from cacao growers in Grenada, 

 St. Lucia, or Dominica. Its occurrence in Trinidad was 

 made the .subject of a paper by Mr. J. B. Borer in the 

 Proceeding's of the Agricultuyal Society, Vol. XVII. 



The algal organism responsible is best known and has 

 been closely studied as the cause of a troublesome affection of 

 tea twigs in India and Java, known as red rust. The alga is 

 widely distributed and common in the tropics, especially on 

 the leaves of numerous trees. That it can resist severe con- 

 ditions is illustrated by its presence on avocado pear leaves 

 in all but the most exposed situations in Barbados. 



The twigs of cacao are occasionally attacked, and die- 

 back due to this cause is reported to be not uncommon in 

 Trinidad, and is recorded from Jamaica. The organism 

 occurs in fairly extensive patches on twigs and small branches 

 of limes in some localities, forming at the same time spots on 

 the leaves which a^e blackish or rusty red according to the 

 stage of development. It occurs also on mango both in India 

 and the West Indies. 



c.\usATi%'K ORGANISM. Cepliakuros vircscciis, Kunze, 

 (C. mycoidea, Karst., Mycoidea parasitica, Cunn.). The 

 causative organism is one of a group of mostly epiphytic 

 algae, and affords an instance of partial parasitism which 

 outside this genus is exceedingly rare in the algae. The 

 organism is very common on leaves, especially those with 

 a smooth surface and somewhat leathery texture. 



In this type of situation it forms, generally on the 

 upper surface, ontnge or rusty red, roundish or less often 

 irregular, .slightly raised patches up to nearly J,-inch in 

 diampti r As a rule the presence of the alga on leaves 

 causes liMle or no damage. In the case of tea, cacao, liiries, 

 and mango alike, it is in its occurrence on the twigs that 

 destructive powers may be developed. 



The first outward siiin of the disease on cacao shoots is 

 the occurrence during the dry .season, on the twigs of the 

 previous .'reason's growth, of dark purplish or black spots 

 ranging up to A or J-inch in diameter. With the com- 

 ing of the rains these take on a rusty red rolour due to 

 the development of a dense pile of fine hairs bearing tiny 

 globular heads, the sporangia of the alga. The latter when 

 ripe di.'-charge when wetted numerous biciliate zoospores, 

 which afiera short active pt-riod come to rest, and aie capa- 

 ble of geiminanon. The ripe sporangia are capable of being 

 broken "ff and transportid by wind before this discharge 

 cccur.<-. 



A second type of sjmrangiuni, eisentially similar, is 

 lormed in the body of the thallus as it occurs on the 

 surface of leaves. 



The alga is it>elf liable to be invaded by fungus hyphac. 

 producing.' for a lime a lype of ttructure resembling that of 

 n li'.heii. 



N.^TUKK oi THE ATT.MK. On various leaves the alga 

 exists in all conditions between the cou.uion case of mere 

 attachment to the cuticle and complete penetration leading 

 to the development of tlialii (cellular expansions) on both, 

 sides of the leaf. The epidermal eels beneath an al^al patch- 

 are usually, but not invariably, discoloured and dead. The 

 only importance attributed to the occurrence of the organism 

 on leaves lies in the provision of material which can 

 infect the stems. 



Infection of the twigs appears to take place from ger- 

 mination of the zoospores in tlie surface cracks which develop 

 in connexion with the formation of the first layers of bark. 

 The alga pu.shes in among the cortical tissues, successive 

 layers of which are sloughed away. The irritation caused by 

 the presence of the parasite sometimes results in hypertrophy 

 (abnormal swelling) of/ the twigs. In severe cases the 

 separate patches first occurring on the twig may coalesce 

 and competely envelop it for some distance. 



If growth is not sufficiently vigorous to throw oti' the 

 infection by new and deeper bark formation, the result is the 

 failure of the leaves and the death of the twig, and short of 

 this effect, the lesions in the bark are liable to afford access to- 

 the die-back fungus (Diplodia). 



INFLUE.M'K OF EXTERN VI- CONDITIONS. The aCCOUntS of 



the diseases caused by this organism agree as to the depen- 

 dence of serious injury on lack of vigour in the host. On tea 

 the disease occurs suddenly and const;intly whenever bushes 

 become unhealthy from any cause whatever. On cacao most 

 harm has been observed where the trees were in poor con- 

 dition as a result of exposure or insufficient drainage. 



The reasons for this are believed to lie in the inability 

 of weakly twigs, as indicated above, to get rid of the parasite, 

 and in the greater susceptibility to infection of twigs in 

 which bark formation is weak and slow. It has been observed 

 that at the other extreme, the very i|uickgrowiug shoots pro- 

 duced on tea after severe pruning develop cracks in the bark 

 which are liable to infection. 



CONTROL. Spraying with Bordeaux mixture has proved 

 ineffective against the disea.se, and no direct method of con- 

 trol is known. The appropriate remedy, adopted with great 

 success \r. the case of tea, lies in close attention to drainage, 

 cultivation, and manuring. 



W.X. 



IMITATION COFFEES. 



Tlic agreeable and stinuilating c|ualities of coti'ee ;is 

 a beverage have given it worldwide importance, and this has 

 led to the employment of a number of substances known as 

 coffee substitutes. An interesting account of .some of these 

 appears in the M'ealth of India for February 191t<. It is 

 pointed out that, properly speaking, these substitutes should 

 rather be called imitation coffees, since they do not contain all 

 the qualities which give to coffee its peculiar individuality. 



Coffte contains three essential piiiiciples: caffeone, an 

 aromatic principle developed during roasting: a bitter prin- 

 ciple po.ssessing febrifugal properties; and eaffein, a stimulant. 

 -Many nervous persons are obliged to forego coffee as a drink, 

 even though they may be fond of it, on account of the 

 presence of catlein. In recent years decaffeined coffees have 

 been put on the market whirii have been deprived by 

 chemical treatment of the .stimulating effect of the eaffein, 

 still retaining the aroma of the caffeone. 



The imitation coffees do not possess all the [)roperties of 

 coffee, and only vaguely suggest its flavour. They are either 

 mixed with coffee or used alone; tliey all oonlain a bitter 



