Because of the wide spread publicity some years ago on the high susceptibility of lilies to diseases, many gardeners even now have the impression that the “queen” of flowers is difficult to grow. This is certainly not so.
Disease-free and disease-tolerant kinds are now available which can be grown with as little trouble as many other kinds of plants that are far less attractive. Plant pathologists also have a better understanding of the disease-producing agents and therefore are better able to cope with them. New and more efficient insecticides and fungicides further help to assure success.
Cultural Practices Are Important
Certain cultural practices have a direct hearing on the prevalence of disease. A well-drained, well-prepared neutral to slightly acid soil is essential. The importance of air and soil drainage, as well as of sanitary practices, will be discussed later.
Some of the well-known lily growers feel that a fertilizer high in potash makes lilies more resistant to diseases, whereas one high in nitrogen produces soft weak stems that are more readily attacked. There is some evidence which suggests this is the case not only with certain lily diseases but with the fire blight disease of apple and pear as well.
Insect Pests
Several species of plant lice, including the melon and lily aphid, attack lilies. These usually infest the growing tips of the plants and produce distorted leaves and a sticky secretion. if aphids caused no more damage than this their importance in lily culture would be negligible. Unfortunately, these pests – particularly the melon aphids – are vectors or disseminators of one or more virus diseases formerly known by the name mosaic and now by such names as fleck, motile, rosette and streak. For the purposes of this discussion these diseases will be referred to as mosaic.
Before describing the nature of virus diseases, it would be well to outline the control measures for aphids. Malathion dusts or sprays arc extremely efficient aphid killers. Frequent use will keep aphid infestations down sufficiently to greatly minimize the chances of spreading mosaic.
Lily Mosaic
In addition to aphid control, lily growers would do well to learn to recognize mosaic symptoms, because the sooner infected plants are removed, the less are the chances of spreading the disease to nearby healthy plants.
The most prominent symptoms of mosaic are a light and dark green mottling of the leaves and a distortion of the flowers and foliage. In some species the mottling is very pronounced but in others it can be detected only by shading the suspected leaves from the direct rays of the sun. In a few species, including Liliunt croceum, elegans, umbellatum, candidum and tigrinum, there are no clear-cut symptoms. In others, brown flecks may appear on the leaves.
The mosaic virus occurs in all parts of an infected plant except the seed. Thus, unless seed is used, there is danger of carrying the disease over from year to year via the bulbs.
Bulbs from virus-infected plants cannot be distinguished from those of healthy plants, but even if ii were possible to do so, there is no known cure.
One can be reasonably sure of growing mosaic-free plants for a long time, however, by adopting a reliable aphid control program and by a strict roguing (lifting and destroying) of infected individuals as soon as noticed. This is the type of program now followed by commercial growers of lilies – a program which provides gardeners with the best bulbs having a minimum of mosaic.
Gardeners can also put to good use the presently known facts regarding the varying degrees of tolerance of varieties to mosaic. Lillian formosanum, for example, is the most easily infected species and usually (lies within the season it becomes infected. It would be wise, therefore, not to plant it near other species that may be harboring the virus.
Among the other lilies that are extremely subject to mosaic are L. auratum, canadense, candidum, elegans, japonicum, Maximowiezi, rubellum, speciosum, superbunt, tigrinum and umbellatum. Some of these, including L. candidum, elegans, speciosum and urnbellatum,however, are so tolerant of the disease that they live on despite the infection.
The following kinds either escape mosaic infection or are rarely affected by it; L. Browni, Henryi, Martagon.
Botrytis Blight
Of the fungous diseases which affect lilies, botrytis blight is perhaps the most troublesome. It is most serious on Madonna lilies but other kinds, including L. callosum; cernuurn, Maxwill, pardaiinum and testaceum, are also attacked.
Blight first appears as conspicuous, orange-brown, oval spots on the ]eaves. In severe infections the spots coalesce and cause the leaves to blight completely. Infected buds turn brown and fail to open; partially infected ones open to form unsightly distorted flowers.
Lilium candidum, chalcedonicum, Humboldti and testaceum are among the most susceptible species. L. speciosum, Hansoni, Martagon and auratum are less susceptible and L. giganteum and Wilmottiae are somewhat resistant.
The fungus, Botrytis elliptiea, the cause of blight, passes the winter as small, seed-like bodies (sclerotia) in infected fallen petals and leaves and in dead stems. It may also winter in its vegetative stage (mycelium) in the basal rosettes of Madonna lilies.
The blight fungus is favored by high humidities. Therefore, lilies should be planted where the air circulation is good so that the leaves dry off rapidly after rains. Low, shaded spots and massed plantings should be avoided and weed growth prevented.
In the spring, when the plants are dry, all infected plant parts should be gathered carefully and burned or thrown into the garbage can. They should never be relegated to the compost heap.
Several preventive sprays are known to be effective in keeping blight under control. Bordeaux mixture with a good wetting and sticking agent gives good control if applied before the disease becomes serious. Fixed coppers also give control if applied early enough and if the leaves are well covered. Susceptible species should he sprayed once a week from the time the plants are 6 inches tall until just before flowering. Sprays containing the material Zineb are also effective in combating the troublesome botrytis blight.
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