Comparative study on the monocycle of Phakopsora meliosmae-myrianthae and Phakopsora pachyrhizi

Asian grapevine leaf rust, caused by Phakopsora meliosmae-myrianthae, presents symptoms similar to Asian soybean rust, caused by P. pachyrhizi, such as the presence of satellite uredinia and early leaf fall. To shed light on the epidemiological behaviour of these rusts, we compared the monocyclic components of these Phakopsora species and the effects of these pathogens on host photosynthesis over time. Both hosts were inoculated with a 10(4) urediniospores mL(-1) suspension and incubated at 23-25 degrees C. Incubation and latent periods were simultaneous at 8 days in Asian grapevine leaf rust and 13 days in Asian soybean rust. The minimum infectious periods were 21 days for P. meliosmae-myrianthae and 13 days for P. pachyrhizi. The continuous production of urediniospores through a long infectious period was observed for both pathogens. Both Phakopsora rusts showed an increase in rust severity with similar progress rates to those estimated with the monomolecular model as 0.07 day(-1) and 0.02 day(-1) for grapevine and soybean rusts, respectively. No increase in the number of lesions was observed from the first assessment, although the number of uredinia increased over time. P. meliosmae-myrianthae and P. pachyrhizi infection reduced the net photosynthetic rates by 22% and 5%, respectively, before the onset of symptoms. Both pathogens withstand non-ideal environmental conditions for secondary infections. We concluded that, although P. meliosmae-myrianthae has a restricted host range on Vitis sp., unlike to P. pachyrhizi with a broad host range, the epidemiological behaviour of P. meliosmae-myrianthae is in some ecological aspects similar to that of P. pachyrhizi.