The incubation period of beet yellowing viruses in sugar-beet under field conditions

Abstract

In three years field trials, the incubation period, i.e. the time between infection and the appearance of symptoms, of beet yellows virus (BYV) and beet mild yellowing virus (BMYV) increased with later infection during the growing season. The incubation period of BYV, a closterovirus, increased from 3 weeks in young plants infected before canopy closure, to 9 weeks in old plants infected in August. The incubation period of BMYV, a luteovirus, increased from 4 to 5 weeks in young plants to 9 weeks in old plants. Symptoms were observed c. one week earlier on the inoculated leaves than on those infected systemically, except on young BYV-infectecl plants. On these plants, symptoms developed in 3 weeks on both leaf types.The incubation period decreased at increasing temperature, a fixed temperature sum being required for the development of symptoms on plants of a certain age. This temperature sum increased with plant age. Symptom development was related to leaf growth; the systemic symptoms appeared after the infected leaves attained their final size. Young, expanding leaves did not show symptoms. Thus the development of symptoms seems to be related to physiological conditions occurring only in full-grown leaves. A low rate of leaf expansion may constitute the underlying reason for the long incubation period of virus symptoms in old plants and at low temperatures.The incubation period was not substantially affected by: (I) the number of Myzus persicae used to inoculate the plants, (2) the number of leaves inoculated, (3) the development stage of the inoculated leaf and (4) the source plant of BMYV, beet or shepherd's-purse, Capse/la bursapastoris. The incubation period can be used to obtain rough estimates of the infection-elate of individual plants, given the date on which symptoms appear.

Retrospective estimation of the data of infection with beet yellowing viruses in sugar-beet under field conditions

“…The number of plants inoculated varied from 450 in June, when the inoculation conditions were varied (Van der Weft et al, 1989), to 60 in July, when only standard inoculations with BYV and BMYV were made. The number of plants inoculated varied from 450 in June, when the inoculation conditions were varied (Van der Weft et al, 1989), to 60 in July, when only standard inoculations with BYV and BMYV were made.…”

“…The viruses were maintained in beet as described before (Van der Weft et al, 1989). Viruliferous aphids were collected from infected beet plants in the glasshouse in May and July 1985.…”

“…Watson et al, 1951Watson et al, , 1975Heathcote, 1986). Research into the within-season build-up of the disease has, however, been hampered by the variability of the incubation period under the influence of growing conditions and plant age, andby the lack of accurate estimates (Van der Weft et al, 1989). Therefore, it was hitherto impossible to relate the population dynamics and the behaviour of vector aphids to the subsequent increase in the number of yellowed virusinfected plants in the crop.…”