Shoot-tip grafting in vitro (STG) (Navarro, et al., 1975) has been used to recover citrus plants free of psorosis and concave gum viruses Roistacher et al., 1976;Roistacher and Kitto, 1977). The percentage of plants obtained free of these pathogens ranged from 0-100, and averaged 69. STG is used in the Citrus Variety Improvement Program in Spain (CVIPS) (Navarro, ,1977. Most trees chosen for this program contained pathogens that caused psorosis-like leaf symptoms when graft-inoculated into Pineapple sweet orange and Dweet tangor seedlings. The exact nature of these psorosislike pathogens (PLP) could not be determined from field and greenhouse symptoms. Attempts to recover PLPfree plants by STG initially failed with many varieties and with others less than 10 per cent healthy plants were obtained (Navarro et al., 1980).In this paper we describe the influence of growing conditions of shoot-tip source plants on the percentage of PLPfree plants obtained by STG. We also discuss the possible separation of several P L P by STG.
Citrus vein enation virus (CVEV) is widely distributed in Spain and circumstantial evidence suggests a high rate of natural spread. No experimental transmission was achievedin extensive trials with several aphid species using a 2-day acquisition period and a 2-day inoculation period. In further experiments, using a 5-7 day acquisition period and a 9-16 day inoculation period, a 95% transmission efficiency was obtained with Aphis gossypii and 10% with Myzus persicae. Furthermore, a 10% transmission efficiency was achieved with viruliferous A. gossypii maintained on healthy plants for a period of up to 14 days after acquisition then given an 11-day inoculation access feed. These results show that CVEV is persistently transmitted by A. gossypii in Spain, thus explaining the high rate of natural spread in the field.
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