EFFECT OF MAIZE STREAK VIRUS-DISEASE ON THE GROWTH AND YIELD OF MAIZEAS INFLUENCED BY VARIETAL RESISTANCE LEVELS AND PLANT STAGE AT TIME OF CHALLENGE

Field experiments were conducted from 1989 to 1991 at Ibadan, Nigeria, to assess effects of maize streak virus (MSV) disease on growth and y ield of maize varieties having different levels of disease resistance. MSV disease reduced yield and growth in all years, but varieties diff ered significantly in amount of loss, disease severity and incidence. MSV disease was negatively correlated with plant height and dry weight , grain weight per plot, 1000-grain weight, ear length and diameter. I n 1989 MSV disease decreased yield of resistant variety TZB-SR by 1.5% , of resistant hybrid 8321-21 by 10%, and of moderately resistant hybr id 8329-15 by 17%. Yield of susceptible variety TZB Gusao was reduced significantly more, by 71%. Plant age at time of virus challenge had s ignificant effects on yield and growth characters, with earlier infect ion resulting in greater disease severity and yield reduction. A signi ficant interaction between variety x age at challenge was also detecte d, indicating that varieties were differentially affected by MSV in re lation to the growth stage when challenged. Disease incidence after ch allenge was lower for the most resistant varieties. This property of l ower disease incidence under equal challenge opportunities (tolremicit y) is an important aspect of resistance. The resistant varieties discu ssed here were bred for tolerance - good yield performance when diseas ed -, but TZB-SR and 8321-21 also exhibited tolremicity. Tolremicity c ombined with tolerance constitutes the overall disease resistance of a variety to a systemic pathogen such as MSV.