KEPORT ON BANANA DISEASE PREVALENT AT ALEXANDRIA. 8^7 



may be made. It has been seen that all the different species pass a 

 certain period of their life-history outside the plants themselves, that is 

 to say, in the soil, this being a common feature in the history of all 

 parasitic animals, since it is the only means by which they can spread. 

 The time, therefore, in which to institute an attack is when the majority 

 are found in the soil ; any attempt to reach the pest when within the 

 plant must be doomed to failure, for it is then in perfect security. In 

 countries where there are well-defined seasons with great differences 

 between them, it is more easy to ascertain exactly the different stages 

 than is the case with such a climate as that at Alexandria, where probably 

 development goes on steadily; that is to say, the free worms are always 

 present in the soil. It is, on the other hand, also very likely that their 

 numbers become considerably increased at certain periods in connection 

 ■with the subsequent generations. Any remedy to be applied would there- 

 fore have its maximum effect only if applied during these periods. This 

 matter can, however, only be definitely decided when the life-histories of 

 the species have- been followed throughout. Experiments could then be 

 conducted as to the most suitable means to employ. In coping with 

 nematodes attacking the beet crop in Germany, a method was successfully 

 adopted which may be mentioned here. Nematodes are found in, one 

 might say, almost every plant in small numbers. Practically all nema- 

 todes living as parasites on plants are not exclusively parasitic on one 

 individual species, for if they find the necessary favourable conditions for 

 existence they will attack another host. Just as a human being or an 

 animal can carry a tapeworm or other parasite without apparent injury to 

 health, and only show signs of suftering when the number increases, so 

 within certain limits can plants withstand nematodes, and only show 

 signs of disease when their numbers become excessive. 



To combat the pest in the sugar-beet plantations, other plants which 

 were suitable as hosts were used to attract the pest. The seed was sown 

 early in spring, some weeks before the beets were planted. The larvfe of 

 the nematode hibernated freely in the soil and attacked the newly sown 

 plants, which were subsequently removed and destroyed. There w^ere thus 

 removed from the soil vast numbers of the pest which would otherwise 

 have attacked the beet. This did not result naturally in a complete 

 clearance of the pest, but the beet was enabled to resist the number which 

 remained. The adoption of this method in the case of Bananas would 

 require certain modifications, but something might be done in this 

 direction ; and then, by providing the plants with suitable conditions for 

 recovery, such as good cultivation and an application of suitable manure, 

 they may recover. 



The idea has been expressed that the disease is one of recent introduc- 

 tion, but this does not seem probable. Species of the genus Heterodera 

 were found by Dr. Looss in a garden at Alexandria some years since, 

 and these were similar to the Heterodera of the Bananas. It is probable 

 that they have now found a most suitable host in Bananas, and have 

 consequently rapidly increased in numbers. They have probably been 

 living in Banana plantations for some considerable time, and the result of 

 years of increase has only now become very apparent. 



Experiments in the direction indicated should be attempted, first, to 



