Comparative epidemiology of grapevine and soybean rusts

Author(s)

Isabela Vescove Primiano

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2019-07-01

Examining board members

Lilian Amorim; Armando Bergamin Filho; Ivan Herman Fischer; Cláudia Vieira Godoy

• Advisor: Lilian Amorim

Abstract

Phakopsora meliosmae-myrianthae, a causal agent of Asian grapevine leaf rust, and Phakopsora pachyrhizi, a causal agent of Asian soybean rust, cause severe epidemics on their crop hosts. Both Phakopsora spp. seem to behave differently to other rusts, i.e. showing a high frequency of pustules on leaves concomitantly with host tissue necrosis and leading to premature defoliation. To shed light on the epidemiology of these rusts, this study aimed to: (i) compare the colonization progress of P. meliosmae-myrianthae and P. pachyrhizi on their hosts, by determination of fungal biomass via histological staining and quantitative polymerase chain reaction (qPCR); (ii) compare the effects of Phakopsora spp. on host photosynthesis and the monocyclic components: incubation, latent, and infectious periods, number of uredinia, and disease severity over time; and (iii) estimate the relative defoliation rate caused by Asian grapevine and soybean rusts and its relationship to a range of disease severity. All pathosystems showed lesion expansion. Mycelial colonization did not extend beyond the lesion border. No increase in the number of lesions was observed over time, but formation of new uredinia of P. pachyrhizi and P. meliosmae-myrianthae within an existing lesion, without the need for a new infection site, increased by 9- to 19-fold, respectively. Incubation and latent periods were coincident for 8 days in Asian grapevine leaf rust and 13 days in Asian soybean rust. Minimum infectious periods were 21 days for P. meliosmae-myrianthae and 13 days for P. pachyrhizi, and both pathogens presented several sporulation peaks. Both Phakopsora rusts showed an increase in disease severity during monocycle, with similar progress rates that were estimated with the monomolecular model as 0.06 and 0.05 day-1 for grapevine and soybean rusts, respectively. P. meliosmae-myrianthae and P. pachyrhizi infection reduced relative photosynthetic rates by 22% and 5%, respectively, before the onset of symptoms. Defoliation rates of grapevine and soybean rusts were positively correlated with mean disease severity, according to a logarithmic model. On symptomless grapevine and soybean leaves, defoliation rates were 0.05 and 0.06 day-1, respectively. On diseased grapevine leaves, defoliation rate was 0.13 day-1 for leaves with disease severity between 12.1% and 25%. The rate of defoliation on soybean leaflets was 0.12 day-1 when disease severity was between 25% and 60%. Our findings showed that the epidemiological behaviour of P. meliosmae-myrianthae is similar to that of P. pachyrhizi. The continuous increase in the number of uredinia within lesions ensures an infectious period with several urediniospore production peaks, which is an efficient survival mechanism for these pathogens in the tropics. This might be directly related to the frequent epidemics caused by Phakopsora spp. (AU)

• FAPESP's process: 15/26108-8 - Aggressiveness of Phakopsora: a comparative analysis of grapevine and soybean rusts with emphasis to histopathology and epidemiology
• Grantee: Isabela Vescove Primiano
• Support Opportunities: Scholarships in Brazil - Doctorate