Determining the best density of Medicago sativa cv. Yazdi for the prevention of alfalfa witches broom in Yazd province.

Alfalfa witches' broom (AWB) is one of the most important alfalfa diseases in Iran. To evaluate the best density of Medicago sativa cv. yazdi for the prevention of AWB, during 2006-2008, different alfalfa densities were studied in a randomized complete blocks design at three replication. Experimental treatments were customary solid planting (YSCS), double seeding solid planting (YSDS), Row planting with distance between rows 20cm (RP20), 40cm (RP40) and 60cm (RP60) and sparsed row planting with 15 (SRP15), 35 (SRP35) and 55 (SRP55) plant per each square meter. Sampling were carried out in 3rd year alfalfa stand and infection percent, disease severity, plant number per each square meter, mean dry and fresh weight, height and leaf shoot ratio of infected plants were compared with healthy plants. In each plot sampling were conducted randomly at two points within a 1 m quadrate. Infection of samples was based on characteristic disease symptoms, ELISA and DIBA test using polyclonal antiserum and polymerase chain reaction using P1/P7 universal primer pair. Based on the result of this study maximum and minimum infection rate of alfalfa witches broom were 55.78 and 5.76 in SRP15 and RP20 respectively. Infection percent were reduced with increasing of plant density. The variation of disease severity were correlated with infection percent and has no significant differences in 5% level except SRP15. Infection percent, disease severity and Fwi/fwh (fresh weight of infected plant to healthy plant) has significant differences in 5% Duncan's test. Among experimental plots, fresh and dry yield of RP20 were maximum and infection percent and disease severity were minimum. The most plant density were observed in RP20 with 169.19 plant/m2 and fresh and dry weight of this treatment were 2996.95 and 806.33 g/m2 respectively. Based on the results of this study, RP20 is recommend for the prevention and reducing losses of AWB.