SummaryThe effect of co‐infection by maize streak virus (MSV) and maize stripe virus (MStV) on plant growth and grain yield was investigated in a susceptible variety of maize (Zea mays), ZS 5206, in Mauritius. Under natural conditions MSV, transmitted by the leafhopperCicadulina mbila, was normally established before MStV, which is vectored by the planthopperPeregrinus maidis;as a result, MStV symptoms were often partially or completely masked by those of MSV, making MStV detection by symptomatology very unreliable. MSV and MStV were diagnosed by ELISA and MStV by a novel method of detecting the MStV‐coded non‐capsid protein.The maize hybrid ZS 5206 was inoculated with either MSV, MStV or both, at two stages in the growth cycle (3–5 or 7–10 leaf stage). A greater reduction in plant growth was observed in plants inoculated singly with MStV (80% and 29% for first and second stage, respectively) than with MSV (50% and 23%, respectively). No cobs were produced by plants singly infected with MStV at the first stage, or co‐infected with MSV and MStV at both stages; however, marginal grain production was recorded in plants singly infected with MSV at the first stage (91% reduction), or infected either with MSV or MStV, at the second stage (65% and 80% reduction, respectively).In maize hybrid ZS 5206, MStV is more virulent than MSV; co‐infection by both viruses causes greater reductions in plant growth and grain yield than single infection by either virus at a given stage of plant development. In the event of co‐infection by MSV and MStV, yield losses can be erroneously attributed to MSV only if the symptoms of MStV are masked by those of the former and if adequate methods for MStV detecti