In susceptible hosts, MSV can be very severe: the photograph below is of a susceptible
commercial Kenyan hybrid maize from the farm of Mrs
Pauiline Ruiru in the Githungiri area near Nairobi, from a heavily-infected (>90%
incidence) field. This plant is very unlikely to make a proper cob, and will be
severely dwarfed. Note the wide, continuous, parallel yellow/white streaks
on all the visible leaves: this would rate a "5" on the breeders'
rating scheme (out of 5) for severity of infection. Note also the intercropping with
beans: this is typical of small farms in Kenya.
(click on image for full-size picture: this is in jpeg format, so should load quickly)
The next photograph shows a moderately tolerant foreign company-supplied hybrid maize of the same age, from the same farm, from a field immediately adjacent to the severely-affected one, but with an apparently lower (approx. 70%) incidence of infection. The plants are far less affected by the virus: plants are larger, and streaks are narrower, and more broken. These plants would rate "3" or less on the severity scale, and may even grow away from the symptoms to some extent. These plants should give a yield, albeit reduced to some extent.
(click on image for full-size picture)
The farmer's response to this disease was to blame the Kenyan seed for being inferior, whereas it was only lacking in being MSV susceptible, and was probably better suited to the climatic region: however, the fact that she would have had to replant all of her maize if she had planted only the local variety, was undoubtedly more important to her than keeping faith with the local seed suppliers. In this case, the Kenya Agricultural Research Institute personnel working on MSV disease actually paid her to keep the diseased plants in the soil so they could follow the effects of the virus on yield and disease spread, so she did not lose. Others are not so lucky, and it is to them that current and future efforts in research on this virus should be dedicated.
(click on image for full-size picture of Pauline Ruiru)
Link to more detailed pictures:
Ed Rybicki wishes to thank all of the KARI staff and all those involved in the ISAAA project on MSV (especially Peter Markham and Florence Wambugu) for the opportunity to get to Kenya, and to see the disease problem at first hand.
copyright Ed Rybicki, November 1997, April 1999
(unless otherwise stated)