Assessment of Stylosanthes seed yield reduction caused by phytoplasma-associated diseases

The effect of Stylosanthes little leaf disease on Stylosanthes scabra cv. Seca seed yield was monitored over 2 consecutive seasons at Southedge Research Station in northern Queensland. The time of initial symptom expression was recorded during weekly assessments to determine what effect time of infection after planting had on seed yield. First flowering date was recorded to establish whether the developmental stage of the plant, as indicated by the onset of reproductive activity, at the time of symptom expression influenced the effect of Stylosanthes little leaf disease on yield. At the end of each season, both diseased and asymptomatic plants were harvested and seed yield determined. Seed yield data from the 1999 season showed that there was no significant difference between the mean yield of symptomatic plants, regardless of when they first showed symptoms, and that of asymptomatic plants. However, during the 2000 season plants that showed symptoms early in the season had a significantly lower seed yield than both asymptomatic plants and plants that became diseased later in the season. This decrease in productivity amounted to a yield loss of 98.8 and 56.5% when the plants showed symptoms at 79�and 110 days, respectively, after planting. If plants became diseased within 30 days of first flowering, they did not produce significant amounts of seed. Yield remained low even when the time between first flowering and initial symptom expression increased up to 60 days, after which yield was extremely variable but within the range of that observed for individual asymptomatic plants. It is concluded that Stylosanthes little leaf disease has little or no effect on seed yield if plants have been flowering for about 8 weeks before symptom expression.