The spread of beet yellows and beet mosaic viruses in the sugar-beet root crop. II. The effects of aphid numbers on disease incidence

Multiple regression analysis of the data described in a previous paper identified alate Myzus persicae of the spring and summer migrations as the most important factor affecting spread of beet yellows virus in the sugar‐beet root crop in England.

Apterae of M. persicae spread little virus, and the contribution of alatae and apterae of Aphis fabae was negligible.

A simple mathematical model of the spread of infection was developed. Assuming that the crop is visited by N aphids at a time when the proportion of plants infected is k0, the predicted proportion of infection for a time 3–4 weeks later (k1) is k1=k0+ 100(1‐k0) (1‐e‐N1), where I=p[(1‐k0)t+k0t‐1]/k0.

This formula adequately accounts for the observed spread of infection when N=1/10 sticky trap count for the 3–4 weeks preceding the time when k0 infection is observed; p= probability of infection by a single aphid =½; t= number of movements per aphid effective for spreading beet yellows virus = 5.

The good fit of this formula to the observed data supports the results of the regression analyses in showing that alate M. persicae were mainly responsible for spreading beet yellows virus. Once infection had been introduced into the root crops in the spring spread was mainly within fields, or between fields in the same neighbourhood.

In areas where beet and mangold seed crops were grown intensively some infection was spread to the root crops by summer migrants from infected seed crops. Apart from this, the greater prevalence of yellows in seed crop areas was caused by the greater development of M. persicae infestation of the root crops which occurs in these areas.

The proportion of plants infected by spring migrants entering the root crops before the end of June was small and variable. It did not vary significantly between seed and non‐seed areas. This implies either that initial infection came equally from sources other than seed crops, or that, if the seed crops were the main sources of infection, winged aphids acquired infectivity by visiting them during migration, and later became very widely dispersed.

In contrast to yellows, mosaic virus spread mainly in the neighbourhood of seed crops within the seed‐crop areas. No significant relation between aphid numbers and increase of infection was established, but there is a strong suggestion that alatae of M. persicae and A. fabae spread the virus, and that the contributions of both species are equal. This suggests that mosaic virus is not spread by movement of aphids within the root crops, but only by infective migrants coming from outside sources. As the virus is non‐persistent the sources must be near to the root crops, and as there is little internal spread many infective migrants are needed to cause a high level of infection. The seed crops fulfil these conditions as they are often severely infected with mosaic, and both vector species breed on them. The irregularity of the data relating aphid numbers to mosaic incidence is probably caused mainly by individual variation in the intensity of infection of seed crops, and in the proportion of migrants caught on the traps which actually derive from them.