ASEPSIS for Plants plants from Abroad
The dangers attending the introduction of foreign countries have in recent from years become so painfully apparent that it has led to the erection of numerous quarantine barriers and even raised the question as to whether the benefits from such introductions are commensurate with the risks involved. There are many weighty reasons, however, for continuing a guarded interchange of plant material between different parts of the world. The plant breeder, in particular, has a vital need for new introductions, especially for the wild relatives of cultivated plants. It thus becomes essential to work out an improved technique for the care of importations, and an excellent start has been made in connection with the handling of citrus plants introduced from the Old World. The occurrence of citrus canker, an insidious bacterial disease the eradication of which in the Gulf States in recent years has cost several million dollars, made necessary special precautions in handling citrus material, precautions which appear easily adaptable to other bud-propagated plants.
The first step is the construction of an insect-proof propagation house, with specially designed ventilator screens and oil moats to prevent the entrance of insects, as well as to see that no insects introduced with the plants are allowed to escape. This is vitally important, since insects are often the active spreaders of disease. The second step is the adoption of a system of “aseptic” plant, propagation. This involves as a matter of routine the disinfection of clothes, tools, and person on each visit to the house; but the new feature evolved for citrus propagation is the double transfer of buds from all original plants. As received from abroad. all plants are disinfected by fumigation or otherwise and placed in a metal “knock-down” screened cage (fig. 12) where they are held until new growth is made suitable to use as bud wood. If this new growth is entirely free from infection or infestation, buds are taken and inserted, by budding or grafting, on vigorous home-grown stocks held for that purpose in the “isolation ward” of the quarantine greenhouse.
As soon as these new buds are safely established and growing, the original plant, with any adhering soil, is placed in a covered container (fig. 12), transferred to a furnace and completely destroyed. The screened cage, readily taken apart, is sterilized with live steam or dry heat before being used again. When the budded plant has made sufficient growth, butts are taken from it—a second transfer— and inserted on new, clean stocks. If these second budded plants prove to be free from infection, they are admitted to the propagation bench of the main quarantine house; but before being sent out for field trial they must still be subjected to a lengthy period of detention and rigid inspection. No plant that has not an absolutely clean bill of health is ever released from the quarantine house.
The aim of this procedure is to make certain that no portion — root, branch, bark, or bud—of the original imported plant is ever released from quarantine. Only new, clean plants, "regenerated” by bud transfers, come forth after this vigorous regimen. As a result of this special equipment and procedure, the citrus-quarantine greenhouse, instead of being a sort of plant “pest house,” as it might easily become, is as nearly absolutely clean in a horticultural sense as it is humanly possible to make it. No system, of course, is proof against personal carelessness and ignorance, and success in such work presupposes intelligent management. So far as the expense of insect-proofing the house is concerned, it has been found that the added thrift of the plants, freed from insect depredation, more than offsets the cost and trouble of installing the necessary equipment. And as modern medicine has developed a system of aseptic surgery, so must modern horticulture recognize aseptic propagation, in a liberal sense, as possible and vitally necessary in dealing with imported plants.
