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Young Philodendron selloum seedlings develop systemic vein chlorosis 2-3 weeks after inoculation followed by severe mosaic and deformation of subsequently formed leaves. Stoner (1964) reported that diseased P. selloum seedlings grew slowly, and that parenchyma and chlorenchyma of diseased leaves possessed fewer than normal starch grains. Hypertrophy and hyperplasia of chlorenchyma, collenchyma and parenchyma occurred in rugose areas of infected leaves. Foliar mosaic symptoms often appearing as localized "feathering" patterns of tissue along the veins are observed on Caladium hortulanum, Colocasia esculenta, Philodendron spp., or Xanthosoma spp. (Buddenhagen et al., 1970; Wisler et al., 1978; Zettler et al., 1978). A single plant may have leaves with striking pale or whitish calico feathering, some with severe or slight veinbanding and others with no visible symptoms. Symptoms of dasheen mosaic potyvirus in Richardia, Zantedeschia and many cultivars of Dieffenbachia include severe leaf distortion and reduction in the lamina but are usually much less evident on Aglaonema and Spathiphyllum (Buddenhagen et al., 1970; Zettler et al., 1978).
Expression of symptoms is often intermittent or seasonal. Alconero and Zettler (1971) reported the alternation of symptomatic and asymptomatic leaves on the same C. esculenta plants, and Zettler and Hartman (1987) reported that Dieffenbachia leaves with symptoms are produced most often during spring and autumn. Dasheen mosaic virus is rarely found in certain Dieffenbachia cultivars such as Dieffenbachia x memoria and Dieffenbachia x bausei because they are hypersensitive to this virus and diseased plants are self-eliminating (Chase and Zettler, 1982).
Infection or re-infection of greenhouse-grown ornamental aroids can be prevented by the selection of virus free stock and use of standard insect control practices (Wisler et al., 1978; Zettler and Hartman, 1987). Although dasheen mosaic potyvirus is apparently not seed transmitted (Zettler et al., 1978) virus control through seed production is impractical and poses many problems. Many aroids are protogynous, and stigmata are only receptive for short periods making pollination difficult. Furthermore, horticulturally desirable varieties do not flower or flower infrequently and seed only remain viable for short periods of time. Lastly, many aroids do not breed true, thus seedlings exhibit a considerable amount of phenotypic variation (Hartman et al., 1972; Zettler and Abo El-Nil, 1979; Volin, 1980; Volin et al., 1981). No true genetic resistance to dasheen mosaic potyvirus has been reported.
Given that dasheen mosaic potyvirus is largely confined to the Araceae, it can be controlled if dasheen mosaic potyvirus-free field-grown aroids are grown in isolation from other aroid crops (Zettler and Hartman, 1986). Unfortunately, given the lack of economic incentives to use pathogen-free planting stock (Zettler et al., 1991), dasheen mosaic potyvirus-free field-grown crops such as Caladium, Xanthosoma, and Colocasia, are not readily available. Likewise, isolated fields are difficult to locate in many commercial production areas. Nevertheless, rogueing of infected plants and isolation of fields as much as possible can delay infection/reinfection for several years (Zettler and Hartman, 1986).
Virus-free propagating units can be harvested from some of the vining ornamental aroids such as Philodendron oxycardium by maintaining them under optimal growing conditions and selecting asymptomatic cuttings (Wisler et al., 1978). Accelerated new growth of dasheen mosaic potyvirus-infected plant tissue may be free of virus thus providing a short period in which cuttings can be harvested. However, inasmuch as symptom expression is often seasonal, cuttings should be observed over a period of several months to confirm their virus-free status (Wisler et al., 1978).
Foliage aroids, many of which were ubiquitously infected with dasheen mosaic potyvirus, are now virtually virus free (Zettler and Hartman, 1987; Shishida et al., 1991). While these crops have a relatively high cash value making tissue culture economically feasible, the same is not true for caladiums and the edible aroids. These crops are easily propagated by tissue culture (Hartman, 1974; Gomez et al., 1989; Hu et al., 1994); however, the current low cash value and limited distribution of these crops makes tissue culture uneconomical (Zettler and Hartman, 1986, 1987; Zettler et al., 1991).
Due to a greater per unit value, most yield studies have been conducted on ornamental rather than edible aroids. However, Ramirez and Gamez (1984) reported significant yield reduction in both Xanthosoma sp. and Colocasia esculenta in Costa Rica. In yield studies on Dieffenbachia maculata 'Perfection', an average of 18.8 cuttings per plant were obtained from 30 healthy plants compared to only 10.1 from 60 dasheen mosaic potyvirus-infected plants. Of the 564 healthy cuttings, 98.4% were judged to be saleable compared to only 47.3% of the 606 infected cuttings (Zettler et al., 1980). In a similar study, Chase et al. (1981) reported that dasheen mosaic potyvirus-infected D. maculata 'Perfection' plants produced an average of 10.1 cuttings per plant, while virus-free 'Perfection' plants produced an average of 26.1 cuttings per plant. Cuttings produced by virus-free plants were judged saleable throughout the study, while those from virus-infected plants were not saleable (Chase et al., 1981). Hartman and Zettler (1974) also found that fresh shoot weights and leaf areas of 27 infected D. picta plants were 63% and 66% less than those of controls. Likewise, respective total fresh plant weights and leaf areas of 36 infected Philodendron selloum plants were 30% less than those of the controls.
Wisler et al. (1978) reported that symptomatic dasheen mosaic potyvirus infected Philodendron oxycardium plants were significantly less vigorous than their symptomless counterparts. Average leaf numbers, leaf areas and vine lengths of diseased plants were 38.6, 31.6 and 65.6% less than those of healthy plants, respectively. Fresh plant weight and leaf areas of infected plants were 30% less than those of the healthy plants, and fresh weight of dasheen mosaic potyvirus infected P. selloum plants was reduced by about 80%.
Yield losses of Caladium hortulanum were also reported by Hartman and Zettler (1974). Fresh corm weights of 69 infected plants were 40% less and leaf areas were 53% less than respective healthy plants. Similarly, Knauss et al. (1975) reported that total corm number of progeny resulting from tissue cultured stock exceeded that of commercial stock (499 vs. 208). Furthermore, sprouting of tissue cultured stock was more uniform than that noted for commercial stock.