Update, Priorities and Progress in Argentina

P. oryzae is a new fungal problem on wheat plants that requires global attention. The disease, known as “Brusone” or wheat blast, is recognised as a serious threat to wheat worldwide. Although currently confined to South America, P. oryzae is considered a highly destructive pathogen given the yield failures and the significant economic losses that it has caused in countries neighbouring Argentina (Bolivia, Paraguay and Brazil). In Brazil, the disease was first reported in the state of Parana in 1985 (Igarashi et al., 1986), later spreading to all wheat-producing regions (Anjos et al., 1996; Goulart & Paiva, 2000; Goulart et al.. 1990; Igarashi, 1990; Maciel et al., 2014; Piccinini & Fernandes, 1989 and 1990). In Bolivia, the disease was first reported by Barea and Toledo (1996) and more recently in Paraguay (Viedma, 2005). Since the first detection of the disease in Chaco Province, Argentina, in 2007 (Alberione et al., 2008; Cabrera & Gutierrez, 2007), research has progressed as follows: In 2012, wheat plants were found with symptoms compatible with WB during routine monitoring of fungal diseases conducted in Llavallol, Province of Buenos Aires. In 2015, the first international report of the research period 2012-2015 at the CIDEFI was published (Perelló et al., 2015). Efforts to date aim to understand the biology and epidemiology of this new disease (Perelló, 2016); to provide knowledge about the interaction between wheat-PyhcH/arà pathotypes; and to focus attention on the susceptibility of the germplasm currently used in Argentina against the fungal infection at both the leaf and heading (spike) stages. In spite of the lack of previous information, the main actions undertaken to involve the characterisation of the fungus population from the morphocultural, molecular and biochemical point of view. Furthermore, it is necessary to improve knowledge of the aggressiveness of different pathotypes from wheat and weeds grasses and other Poaceae different from wheat and on wheat commercial cultivars currently under field cultivation. Our studies focused on the ability of the fungus as a seed-borne pathogen and its transmission to seedlings. The effect of the fungus infection on a 1000 grain weight reduction and protein content was recently analysed (Perelló et al., 2017; Martinez et al., 2019). Our research also focused on the role of the different components that participate in the cross-talk among hormone-induced signalling pathways triggered in wheat plants infected with isolates of Pyricularia of different aggressiveness under abiotic stress conditions such as salinity (Cabot et al., 2018).

The devastating epidemic of WB in 2016 in Asia (Bangladesh) was a warning of the global threat against other countries free of the disease (Islam et al., 2016). Although WB has not yet been detected in Europe, it is necessary to prepare for the disease. In this sense, a platform of collaboration between UNLP (National University of La Plata, Buenos Aires, Argentina) and two European Universities in Spain and Germany has been initiated. The UIB (University of Balearic Islands, Spain) project aims to understand the fungus behaviour infecting wheat plants under salinity stress and the gene defence expression. Moreover, collaboration with RWTH University (Aachen, Germany) has been initiated with the aim of deciphering the resistance of wheat against the upcoming threat of P. oryzae which causes blast disease. In Argentina, no new reports of field detection of the disease on wheat exist to date. However, the potential for a highly severe occurrence on barley plants under field conditions is of concern (Gutierrez & Cundom, 2015). Moreover, the disease may be masked or confused with other diseases of similar symptoms in spike such as Fusarium head blight (FHB) caused by F. graminearum, or it may go unnoticed due to the lack of a correct diagnosis. All these factors require immediate attention by the researchers and policy makers involved in wheat crop protection to prevent new outbreaks of WB.

Variability in the degree of pathogenicity was tested in artificial inoculation experiments (Perelló et al., 2017; Martinez et al., 2019). Previous documented experiments under greenhouse conditions (Perelló et al., 2017; Martinez et al., 2019) showed significant differences in lesion development on leaves (infected area [%]/ total area of leaf) between isolates, and wheat varieties after foliar inoculation with P. oryzae at the tillering stage (Z2.1) (Zadoks, 1972). Moreover, different patterns of reaction across leaves depending on the type of strain inoculated were found, such as minute pinhead-sized spots, small brown to dark lesions with or without grey centres or typical blast lesions with elliptical shapes and grey centres. These results indicate variation in aggressiveness among 15 P. oryzae isolates evaluated on two varieties (Figure 6.4 and Table 6.3). Saprophytic growth and conidiation on the basal senescent leaves under greenhouse conditions were also previously reported (Cruz et al., 2015). Moreover, it was proposed that the lower canopy of wheat cultivars could play a relevant role as a source of initial inoculum for the development of wheat head blast infection and epidemics (Cruz et al., 2015). However, very little information is available in the literature and more research is needed to determine whether the inoculum

Severity of Pyricularia pathotypes isolates evaluated on varieties BioInta 3004 and Baguette 18 at seedling stage

FIGURE 6.4 Severity of Pyricularia pathotypes isolates evaluated on varieties BioInta 3004 and Baguette 18 at seedling stage.

TABLE 6.3

Phenotypic Reaction (according to Valent et al. [1991]) on 20 Commercial Wheat Varieties Examined for Symptoms at Seedling Stage (Z1.4)

Variety/isolate

Phenotypic reaction

Mo33Ar

Mo28Ar

Mo12Ar

Mo6Ar

ACA315

1

3

1

1

ACA360

1

2

1

1

Baguette 11

4

4

4

4

B agüeite 601

1

3

3

3

BASILIO

4

4

4

4

BIOCERES1008

1

3

2

2

Biolnta 3006

1

4

4

4

Biolnta 2006

1

3

3

3

BUCK APARCERO

1

2

4

0

BUCK BELLACO

2

1

1

1

ALGARROBO

0

3

3

3

CAMBIUM

0

3

1

1

KLEIN SERPIENTE

2

3

3

3

KLEIN LIEBRE

4

3

3

1

MSINTA615

4

3

4

3

MSINTABON215

1

4

4

4

SY120

0

3

4

0

SY211

1

1

1

1

TSR1066

0

2

2

2

TSR1146

0

2

4

4

originating from leaves has major relevance as a source of spike infection in severely affected commercial fields.

Concerning the impact of the disease at the spike stage, the infection patterns at anthesis (Z65 stage) (Zadocks, 1972) were evaluated by inoculating different wheat cultivars under greenhouse conditions (Perelló et al., 2017; Martinez et al., 2019). Information on the disease damage indicates highly significant differences in the susceptibility levels of the different wheat cultivars tested. Interestingly, isolates with contrasting behaviour according to the cultivar were found (Figure 6.5). These results indicate that there is an opportunity to identify large numbers of tolerant genotypes.

The seed-borne infection and transmission of P. oryzae on different commercial wheat cultivars currently grown in Argentina were analysed using the standard blotter test method, and differences among the wheat cultivars and isolates tested were shown. Klein Proteo germination was not affected by any of the isolates of the fungus tested; on Baguette 18, isolates PyBra and PyBol caused a significant reduction of 15% in germination. Interestingly, all the isolates tested significantly affected the germination of BioInta 3004 causing a reduction of between 50% and 66%. Symptoms observed were total or partial rot of the seeds, and necrotic symptoms with brown dots and brown or grey spot discolouration. The presence of fungal mycelium was observed and recovered under microscope. The incidence of symptoms indicated that the cultivar BioInta 3004 was the most severely infected. The incidence of 60% of grains with softening symptoms was observed on BioInta 3004 in combination with three of the five tested isolates. Moreover, seedlings that emerged from BioInta 3004 infected with isolate PyArg22 showed symptoms of the disease (necrotic spots on coleoptile and first leaf emerged). Seed transmission was evaluated on superficially disinfected and then lyophylised BioInta 3004 seeds using

Severity of Pyricularia pathotypes isolates evaluated on varieties BioInta 3004 and Baguette 18 at heading stage

FIGURE 6.5 Severity of Pyricularia pathotypes isolates evaluated on varieties BioInta 3004 and Baguette 18 at heading stage.

Transmission from seed to seedling of P. oryzae in wheat A. wheat seeds symptoms and the presence of mycelium B

FIGURE 6.6 Transmission from seed to seedling of P. oryzae in wheat A. wheat seeds symptoms and the presence of mycelium B.

the PCR technique. These results were positive confirmation of the presence of P. oryzae in wheat seeds (Figure 6.6).

Recent results under greenhouse conditions in Argentina indicate that the fungus has the ability to grow and develop below the optimal values indicated in previous references (Com. Pers, data not shown). In vitro assays with two wheat isolates of P. oryzae pathotype Triticum in three different temperatures (15°C, 20°C and 25°C) and four water activity (aw) values (0.995. 0.980. 0.950 and 0.90) showed that the fungus could grow and develop in temperatures of 15°C-25°C and water activity values between 0.950 and 0.995. However, the faster growth rate was observed at the highest temperature and 0.995 wa (Larran et al., 2018).

Regarding the role of secondary hosts in the epidemiology of the disease, Perelló et al. (2015) recovered the fungus from native weed and grasses (Stenotaphrum secundatum, Setaria sp., Eleusine sp., Bromus unioloides, L. perenne, Echinochloa cruz-galli, Avena fatua and Cynodon dactylori). Most of the isolates were pathogenic on wheat plants (Figure 6.7).

It is known that the fungus affects a wide range of weeds normally found in association with cereals such as E. indica, D. sanguinalis, Brachiaria, E. crus-galli, P. setosum, C. dactylon, L. multifiorum, Cyperus rotundus and St. Augustine Grass (5. secundatum [Walt.] Kuntze), some of which have been identified in Argentina as secondary hosts (Martinez, 1971; Wolcan & Perelló, 1987; Pedraza et al., 2008, 2014). Nonetheless, little is known about the importance of the inoculum proceeding from secondary hosts under natural conditions.

In Argentina, results indicate that Pyricularia isolates from native weeds and grasses cause typical symptoms on wheat. Isolates are pathogenic for wheat plants of the cultivars BioInta 3004 and Baguette 18 under greenhouse conditions. Symptoms commonly observed were chlorotic lesions, necrotic diamond-shaped lesions, greenish necrotic lesions and elongated brown lesions, depending on the strains and cultivars analysed (Perelló et al., 2017).

 
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