Sources of Inoculum
Potential sources of primary inoculum for disease development are infected wheat seeds, crop residue or infected wheat stubble, and conidia from grass species other than wheat where the fungus overwinters (Prabhu et al., 1992; Urashima et al., 1999; Urashima & Kato, 1998; Kholi et al., 2011).
Goulart and Paiva (1990) have demonstrated the transmission of the fungus by wheat seed; however, little is known about the importance of seed infection in the epidemiology of the disease (Urashima et al., 1993). The presence of P. oryzae in the seed is directly related to germination and vigor, as this fungus can be transmitted to the progeny causing serious losses to seed producers and farmers (Teixeira & Machado, 2003). Infected seeds are one of the mechanisms of long-distance diffusion, which results in their introduction into other countries, states and regions. When infected, seeds appear small, shrivelled and with a low specific weight (Martinez et al., 2019). Consequently, most of these seeds are eliminated in the process of harvesting, which explains the low incidence of P. oryzae in commercial wheat seed.
Given the importance of the seed as a major source of disease transmission and the origin of the primary inoculum of infection in wheat seeds, WB needs to be considered and evaluated as a priority. In this way, the use of pathogen-free seeds is an important component of globally integrated wheat blast management worldwide.
It is reported that the fungus P. oryzae survives on crop residue (CIMMYT, 2014) and plant debris (Fernandez et al., 2017). It is also reported that P. oryzae does not survive in stubble at the colder temperatures of Northern Indiana (Harmon & Latin, 2005), suggesting poor winter survival. Recent results by Pzolotto et al. (2016) in Brazil have shown that sporulating lesions of Pyricularia on wheat leaves, spikes and stems diminish over time. Moreover, according to their results, sporulation is no longer observed after 140 days of exposure. The authors concluded that it seems highly unlikely that infested wheat residues serve as a source of the wheat blast inoculum for the next wheat crop. The management of crop residues is not a key point to control the development of wheat blast. A strong emphasis should be placed on the presence of other hosts (Pizolotto et al., 2016).
6.4.5 Grass and Weeds as Hosts of P. oryzae
The role of grass weeds as a collateral host of P. oryzae in the wheat system must be considered as a possible reason for the increase in wheat blast. P. oryzae can infect major cereals such as rice, its best-known host, as well as wheat, barley, rye, triticale and oats. Perennial forage crops, such as signal grass, or annual crops, such as oats, which are known to be hosts of the wheat blast pathogen, are present in the landscape and thus provide a reservoir for the pathogen (Marangoni et al., 2013; Castroagudin et al., 2016). From the 21 weed species collected in the field in Paraguay, the fungus was observed on Bromus catharticus, Chloris gayana, Cyperus diffusus, Digitaria horizontalis and Lolium multiflorum, and pathogenicity tests conducted with isolates collected from B. catharticus, D. horizontalis and L. multiflorum confirmed their potential to infect wheat (Chavez & Kohli, 2015). According to Ceresini et al. (2016) in Brazil, isolates of Pyricularia from several poaceous hosts such as Avena sativa, C. echinatus, Cynodon spp., D. sanguinalis, Elionurus Candidas, Echinochloa crus-galli, E. indica, Rhynchelytrum repens and Urochloa spp. were indistinguishable from P. graminis-tritici isolated from wheat, and populations from the two host groups had the same genetic structure. Because Urochloa is a widely grown pasture grass occupying more than 90 million ha in Brazil, they propose that Urochloa provides a major source of wheat blast inoculum and may be the preferred host for pathogen recombination.