The origin of stunt is still a mystery. One can hardly assume that it is a recent import from some other country. Stunt was reported from Australia in 1951 and from Holland in 1952, long after it had become generally distributed in North America. Possibly stunt is endemic in one of the many species of Compositae that are capable of carrying the disease without symptoms, and perhaps it spread to chrysanthemums in recent years, but no evidence can be cited in support of that assumption.
THE SEARCH for stunt and the methods devised for detecting it have brought to notice other virus diseases of chrysanthemum. Some of them doubtless were present before stunt but remained unnoticed or were dismissed as unimportant.
Aster yellows, a common virus disease of many plants, notably plants of the aster family, has been known to affect chrysanthemums since the work of L. O. Kunkel at the Boyce Thompson Institute in 1926. Symptoms in chrysanthemums are variable. When the flowers produced are green instead of the color normal for the variety, aster yellows is clearly present. Sometimes the upper branches of a flowering stem are thin, pale or yellowish, and more upright than usual. Again, a number of thin, weak shoots bearing tiny leaves may arise from the base of the plant.
Aster yellows appears in outdoor chrysanthemums and also in the greenhouse when plants are brought in for propagation or for flowering.
Affected plants are unsalable and commonly die in a few months after becoming diseased. In most sections of the United States aster yellows is infrequent in chrysanthemums, but in places where the virus is common in other host plants and the leafhopper vector, Macrosteles fascifrons, is abundant, a considerable number of garden chrysanthemums may be infected each year.
Chrysanthemum mosaic was known to commercial growers in the variety Good News before stunt appeared. J. R. Keller, of Cornell University, first showed that this mosaic is due to a virus by grafting apparently normal Blanche to Mistletoe. The causal agent, which he called Q virus, is symptomless in Blanche and several other florists' sorts and many garden chrysanthemums. In Blanche the mosaic virus and the stunt virus combine to form a complex known as crinkle or stunt-mottle, with crinkling of leaves and marked stunting. Mistletoe varieties show a yellowish-green vein banding followed by mottling, sometimes with marked reduction of leaf size, and even with necrotic effects such as bud and leaf blasting and dieback of young shoots. Strain variation in the mosaic virus seem to account for these gradations in severity of symptoms.
Cross inoculations showed that the Q virus induced News mosaic in Good News and that News mosaic virus reproduces Q symptoms in Mistletoe. Symptoms in Good News include a. well-defined yellowish-green mottling with crinkling of leaf margins and reduction in vigor of the plant. Viruses of the mosaic type have been demonstrated also in chrysanthemums from England and Denmark. The mosaic virus is manually transmissible but less readily than is the stunt virus. Prevalence of mosaic in garden chrysanthemums and in florists' varieties that have been grown outdoors in summer suggests that natural spread of the mosaic virus occurs in the open, but no vector is known.
Chrysanthemum rosette occurs in apparently normal plants of the Ivory Seagull variety but induces marked dwarfing with yellow vein banding and crinkling in Blazing Gold. The disease was detected in Ivory Seagull and in Mamaru by C. J. Olson, of Yoder Brothers, Inc., and by me in Matador at Beltsville. Variation in intensity of yellowing, crinkling, and dwarfing in the Blazing Gold variety suggested that strain variation occurs in this virus. Manual transmission of the rosette virus is difficult, with one of ten plants infected in each of two trials. No evidence of natural spread has been detected thus far and no vector is known. The Good News variety develops marked rosetting with dull-yellow mottling when infected, and serves to distinguish the rosette virus from the mosaic virus.
The occurrence of other virus diseases in addition to stunt complicates the problem of virus detection by graft indexing. Chrysanthemum mosaic and rosette are much less important than stunt, for they are damaging to few varieties, while stunt is injurious to practically all. Nevertheless anyone taking the trouble to index chrysanthemums for virus disease will not wish to let one of these diseases pass undetected. Blazing Gold provides prompt and distinctive symptoms for stunt and for rosette. Mosaic is expressed by slightly lower vigor, smaller leaves, and occasional distortion of the terminal lobe of the leaf. Mistletoe expresses distinctive symptoms of stunt and of mosaic, but is slow to respond to rosette. Good News offers clear reactions to mosaic and to rosette, and distinguishes those diseases from each other. It expresses stunt as transitory yellow veining. The ideal variety that will express promptly clear and distinctive symptoms for each of the three viruses is not yet known.
Tomato aspermy, a virus disease of tomatoes and chrysanthemums, has been known in England for several years, but was first distinguished from cucumber mosaic in 1949. Affected tomatoes often fail to set seed after infection takes place, hence the name aspermy. Chrysanthemums are commonly affected in England and chrysanthemums are important reservoirs of the virus, which spreads to tomatoes in the mixed nurseries common in England. The tomato aspermy virus infects also tobacco and many other plants but not cucumber. It is carried in the nonpersistent manner, by the green peach aphid.
Additional viruses of chrysanthemums were described in 1952 by Dirk Noordam of the Instituut voor Plantenziektenkundig Onderzoek at Aalsmeer, the Netherlands. He found two viruses common, one of which he classed as a strain of cucumber mosaic virus, and a second which he called virus B. His B virus, infectious to petunia but not to tobacco, is also common in chrysanthemums in the United States. Noordam's chrysanthemum strain of cucumber mosaic virus is similar to the aspermy virus in many respects, but is serologically related to the cucumber mosaic virus. In chrysanthemums it is associated with distortion and changes of color in the flower, perhaps together with virus B. Aspermy and the chrysanthemum strain of cucumber mosaic may be merely two names for the same virus, although Noordam did not reach this conclusion. He did include many English chrysanthemums in his studies; and English workers frequently refer to cucumber mosaic in chrysanthemums. However, regardless of its technical name, this chrysanthemum virus differs in important respects from the cucumber mosaic virus strains common in the United States.
At Beltsville we detected the aspermy virus in 1951 by manual inoculation to tobacco from two varieties of chrysanthemum recently imported from England, from one plant from Denmark, and from the variety Nightingale from Ohio. The virus apparently has been introduced into the United States repeatedly with European chrysanthemums which have been in popular demand here since 1945. In our tests the aspermy virus has infected more than 35 plant species, including tobacco, tomato, pepper, lettuce, spinach, and several ornamental plants and weeds. Marglobe tomatoes were severely diseased and bore some seedless fruits. Floyd Smith found the aspermy virus transmitted in the nonpersistent manner by four species of aphids, the foxglove aphid, the green peach aphid, the black chrysanthemum aphid, and the green chrysanthemum aphid. Aphids transmitted the virus readily from chrysanthemum to tomato and tobacco, as well as chrysanthemum to chrysanthemum and tomato to tomato. The aspermy virus is thus well equipped to persist in perennial plants such as chrysanthemum and to spread to vegetable crops that grow near them in gardens.
At Beltsville we graft-inoculated 13 varieties of chrysanthemum with scions of the Nightingale variety, now known to carry both the aspermy virus and the B virus. The flower distortions described by European workers failed to develop. Most varieties expressed mottling or veining in young actively growing leaves but some never showed clear symptoms. Leaf symptoms were generally masked as the chrysanthemums approached flowering. The variety Good News expressed well-defined mottling without leaf distortion. The B virus, not the aspermy virus, produces these symptoms in Good News. The four species of aphids that transmitted the aspermy virus also transmitted the B virus from chrysanthemum to chrysanthemum. Present evidence indicates that the B virus is more injurious to American chrysanthemums than the aspermy virus; however, the aspermy virus is the one that damages vegetables.
PHILIP BRIERLEY, a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, has been with the Department of Agriculture since 1922, and has studied diseases of various ornamental Plants. He has degrees from the University of Minnesota and Cornell University.