Infection time and density influence the response of sorghum to the parasitic angiosperm Striga hermonthica

Two cultivars of sorghum (CSH-1 and Ochuti) were grown in the presence and absence of the root hemiparasite Striga hermonthica in uniform conditions i n the field in Kenya, Africa. S. hermonthica had a marked influence on grow th and photosynthesis of 'CSH-1'; however,'Ochuti' showed a less severe res ponse to infection and tolerance of the parasite. The variation in genotype response might be partly explained by later attachment of the parasite and a lower level of infection. Laboratory studies were used to determine the importance of both variables in determining host response to infection. Ear ly infection by S. hermonthica had a more negative effect on the host than late infection. The level of parasite biomass supported by the host also in fluenced host productivity but the relationship was nonlinear. Low degrees of parasite infection had a proportionately much greater effect on host gra in weight than at greater parasite loading. Early infection of 'Ochuti' in laboratory conditions resulted in lower stem dry weight than in uninfected plants but not in smaller total plant biomass or lower rates of photosynthe sis. In conclusion, the time of parasite attachment affected host performan ce and might explain much of the variation in host sensitivity both within and between studies. The level of parasite infection affected host performa nce to a lesser extent. In addition, late attachment and low levels of infe ction might have implications for control management strategies.