28 BULLETIN 3 3, PORTO EICO EXPERIMENT STATION 



diameter of about 1 inch grapefruit is not attacked by scab. In view 

 of the fact that many trees have a habit of blooming over long 

 periods, spraying for scab must be done almost continuously where 

 attempt is made to keep all the fruit clean. 



The formula for Bordeaux mixture is usually 3-3-50. Some 

 growers use commercial calcium caseinate as a spreader, whereas 

 others use a paraffin oil-soap emulsion, the quantity of which varies 

 according to the hardness of the water. 



After sprayings with Bordeaux mixture for scab have been dis- 

 continued, sprayings with oil emulsions for scale should be begun 

 and followed with sprayings for rust-mite control. Failure to spray 

 for rust mite has resulted in the shipment of very little bright grape- 

 fruit from Porto Eico. That which is called bright usually con- 

 tains hundreds of rust-mite punctures. Sulphur is the best remedy 

 for rust mite and does the least harm to the foliage and the fruit 

 when applied dry. A dusting machine should be found in every 

 large grove for covering the trees with sulphur after sprayings with 

 oil have ceased. Only the pure sulphur should be used. 



Lime-sulphur (1 gallon, having a density of 32° Baume, to 30 

 or 40 gallons of water) should be applied for scab control when the 

 trees are the most dormant. When they are blooming the strength 

 should be reduced (1 gallon to 60 or 75 gallons of water), and after 

 the fruit has set and the foliage has matured the mixture should be 

 1 gallon to 40 or 50 gallons of water. Lime-sulphur, although a 

 fungicide, does not entirely destroy beneficial fungi but does prob- 

 ably injure them. It is also used as an insecticide against the various 

 scales, and when used frequently enough is effective against the rust 

 mite. In the powdered form sulphur can be applied more cheaply 

 and more frequently. 



The most common root diseases observed in field studies appar- 

 ently are the results of cutting the large roots. Injured roots are 

 the rule rather than the exception in cultivated groves, and it is 

 readily apparent that if infection took place as the result of every 

 injury the root system would be continually diseased. One form of 

 root disease which attacks trees growing in certain areas and spreads 

 from one tree to another is not caused by root injury. This disease 

 has been studied by the pathologist of the station for the past two 

 years, but the causal organism has not as yet been found. Fortu- 

 nately the disease is not very prevalent. 



Foot rot appears on the crown roots and lower part of the trunk 

 and causes heavy losses in most of the groves. (Fig. 16.) Obser- 

 vations indicate that the disease can be prevented by planting 

 the trees on mounds so as to expose the crown roots to the air. 

 Such roots are not attacked, but may readily become infected after 

 they are injured ; hence the practice of planting on mounds involves 

 a strict adherence to the rule of keeping all cultivation away from 

 the tree. In some of the old groves diseased trees have been saved 

 by removing the soil from immediately around the trunk to expose 

 the crown roots. This practice, however, is successful in sandy soils 

 only ; the water accumulating in the excavation during rainy weather 

 would be fatal to trees in clay soil. On land having a gentle slope 

 the excavations could probably be advantageously connected with 

 drainage ditches. 



