,30 T BIN ID AD AND TOBAGO BULLETIN. [XVIII. 1. 



LEAF SPOT. 

 Another disease, somewhat similar to the Black Spot, is that called 

 "Leaf. Spot" caused by the fungus Ccrcosj^ora rosccicola, Pass. This 

 disease, in appearance, is somewhat like the Black Spot, but can be 

 readily distinguished from it because the spots are circular, and have a 

 distinct dark purplish border. The effect of both diseases on the plants 

 is the same, namely, premature and repeated defoliation which gradually 

 weaken the bushes. 



POWDERY MILDEW. 



This disease is very common at certain seasons of the year and is 

 very destructive to certain varieties. 



The fungus causing the disease is known as Splucrotlteca 2^<^'>'osa, 

 Lev. The fungus attacks the verj' young leaves and shoots and covers 

 them with a fine flour-like mildew. 



The affected leaves and shoots become dwarfed, curled and deformed, 

 and soon wither up and die. See lower figure of plate. 



This disease spreads very rapidly, especially during the time of the 

 year when the nights are cool. Picpeated attacks of the disease cause 

 the plants to lose their vitality. 



Treatment. 



All three of the diseases mentioned above yield readily to the same 

 treatment. Experiments liave been carried out in which various 

 fungicides have been used, and though several have given good results, 

 the most successful have come from the use of flowers of sulphur mixed 

 with dry arsenate of lead. This is much easier to apply than liquid 

 fungicides, as it is simply dusted on the plants, and does not discolour 

 the folitige and flowers to the same extent. 



The flowers of sulphiir should be very iine. so that it will pass 

 through a 200 mesh to the inch sieve, and dry arsenate of lead tshould 

 be added at the rate of one pound to every nine of sulphur, and 

 thoroughly mixed in. (D It is necessary to make several applica- 

 tions of this mixture to the plants, especially at the beginning of the 

 rainy season, before the plants become badly infected. It must alway be 

 borne in mind that plant diseases really cannot be cured but only 

 prevented. The numbers of dustings necessary for a season cannot be 

 definitely laid down, but is dependent altogether on weather conditions. 

 At the beginning of the season however, it is advisable to make three 

 or four applications at intervals of from seven to ten days. Later in 

 the season the interval between the dustings can be lengthened 

 according to judgment. 



Daring the past season, several experiments have been carried 

 out ^\ith the sulphur arsenate mixture, as a control for the leaf spots 

 and mildew, and at the time of writing the treated plants are practi- 

 cally free from fungous diseases, and in very healthy condition, while 

 the untreated plants are badly affected, or even leafless. 



(1.) Sulphur and arsenate of lead can be inuxhased locally, hut the i^rices are 

 very high, which makes the treatment rather costly. The IJoard of Agriculture 

 has, however, ordered a mixtitre made 1)3- the Union Snliihur Conijiaiiy, which will 

 be sold at a reasonable rate. I'he Board "also lias on hand a limittd number of J'uut 

 ■<luiiters, which can be used for ppi)lyiiig the mixture. 



