How to prevent damage from zonal spot in rice

Detected in recent harvests in rice cultivation during the seedling and tillering phase, Drechslera gigantea requires increased attention from rice farmers and researchers to prevent losses 

The occurrence of pests and diseases in rice farming has gained importance in recent years. Taking rice production as a reference 12 years ago, disease management was considered secondary. With the increase in grain productivity, crop protection has gained importance. Several researchers highlight that the occurrence of epidemics, as well as the emergence and aggressiveness of diseases, is directly related to the type of cultivar used, the sowing density and management practices in general. In this sense, it is important to pay attention to those diseases that generally cause damage every year, such as bruzone (Pyricularia grisea) and brown spot (Bipolaris oryzae), burning of the sheaths (Rhizoctonia solani) and leaf scald (Gerlachia oryzae), as well as monitoring those with secondary importance.

Since the 2014 agricultural harvest, technicians and producers, both in the central region and on the border of Rio Grande do Sul, have observed the presence of an atypical spot attacking the crop already in the seedling and tillering phase. Since then, several rice plants have been brought to the Phytus Institute's microbiology laboratory for disease identification.

Description of symptoms

The lesions of the disease are normally small oval or circular spots of brown or brown color with a grayish center and a reddish-brown outline (Figure 1). In some lesions, dark-colored globose structures, possibly perithecia, are observed (Figure 2). When at a more advanced stage, these spots coalesce, forming “zones” on the leaves (Figure 3), thus compromising the photosynthetically active area. The onset of the disease can easily be confused with the circular spot (Alternaria padwickii) and bruzone (Pyricularia grisea).

Identification of the causal agent

Initially, two methodologies were used to identify the pathogen - isolation in Potato-dextrose-agar (BDA) culture medium with incubation at 22°C and a 12-hour photoperiod for five days and a humid chamber using leaves with symptoms, under the same conditions already mentioned. . On the fourth day of incubation, it was possible to notice dark, rounded structures in the center of the spots, resembling a perithecium (Figure 2). When examining the structure with the aid of an optical microscope, it was found that it was the teleomorphic (sexual) phase of a fungus. Based on the characteristics of the ascospores, as well as the symptoms, the possibility of the disease being caused by Bipolaris oryzae, Drechslera oryzae ou Alternaria padwickii. After consulting the literature, it was concluded that it was probably the sexual phase of Drechslera gigantea (Pyrenophora gigantea) (Figure 4). After isolating several rice samples, from tillering to the adult plant, it was found that, depending on the cultivar and location, it was possible to identify both the teleomorphic and the anamorphic phases (Drechslera gigantea syn. Helminthosporium giganteum) of the fungus (Figure 5), although there is no tendency for one or the other to occur.

To confirm the causal agent, Koch's Postulate was performed, a procedure that helps identify the pathogenicity of the fungus on a susceptible host. The postulate basically consists of promoting growth conditions for the organism associated with the symptoms and, subsequently, maintaining the organism's pure culture in an artificial medium. For isolation, the transition areas between the infected tissue and the healthy tissue of the leaves were selected, from which fragments were removed with the aid of a scalpel. Subsequently, these fragments were subjected to superficial disinfestation, being immersed in 70% alcohol, 1% sodium hypochlorite and washed three times in sterile distilled water, for one minute in each solution. Afterwards, the fragments were placed on sterile filter paper in the laminar flow chamber for approximately ten minutes to dry. Finally, the fragments were deposited in petri dishes containing PDA culture medium and incubated at a temperature of 23°C and a 12-hour photoperiod, for approximately five days. After this period, the structures were observed under a magnifying glass and optical microscope to identify the pathogen. The reisolation of the symptoms observed in the inoculated plants (Figure 6) confirmed the causal agent as being Drechslera gigantea. The fungus had a brown conidiophore with one to seven septa, conidia measuring 166-485 x 10-29µm, with spore germination occurring either at the base or apex of the spore, but rarely in the middle. In V8 culture medium, the culture presented a cottony appearance with a dark gray to olive color, in accordance with what is described in the literature (Shoemaker, 1959; Ahn, 1980). 

Epidemiology

Because it was identified in rice seedlings, it is likely that, like Bipolaris oryzae, there is transmission via seeds of the disease. Germination of pathogen conidia occurs between 15ºC and 23°C, and the longer the dew period, the greater the severity of the disease (Green et al.

History and importance of the disease

The first occurrence of the disease in its anamorphic phase occurred in 1979 in wild rice in Colombia, Panama and Peru (Ahn, 1980). Subsequently, the disease was identified causing damage in experimental wild rice fields at the University of Minnesota (USA).

In the 2003 harvest D. gigantea was identified in wheat plants, causing numerous circular and oval lesions with a brown margin and gray center (Chowdhury et al.

In cultivated rice plants, it was identified in Venezuela (Rodriguez & Nass, 1990). In the 2006/2007 harvest, the pathogen was found causing spots on cultivars BRS Querência and BRS Atalanta in crops in Capão do Leão, in Rio Grande do Sul, and in the following harvest the disease demonstrated high severity in the final stages of rice development. in the same location (Nunes, 2008).

The pathogen causes zoned spot on banana and grass species, and is also used as a bioherbicidal agent in the control of Setaria viridis of the Poaceae family, a common weed in Canada, resistant to herbicides that inhibit Acetyl-CoACarboxylase and Digitaria sanguinalis in Florida. Drechslera gigantea can cause zoned spots on: Agropyron, Agrostis, Anthephora, Bambusa, Bromus, Cenchrus, Commelina, Cynodon, Dendrocalamus, Echinochloa, Eleusine, Elymus, Eragrostis, Festuca, Glyceria, Hilaria, Lasiagrostis, Leersia, Melinis, Musa, Oryza, Panicum, Pennisetum, Phalaris, Poa, Saccharum, Sporobolus, Teramnus, Trichachne, Uniola, Zinnia (Sivanesan, 1992).

Considering the high incidence of this disease in the 2014/2015 harvest and also considering the large number of alternative hosts of Drechslera gigantea, it is of great importance to better understand this pathosystem, as well as to study its behavior in relation to cultural practices that may interfere in the disease cycle. Furthermore, it is necessary to investigate chemical control, both via seed and aerial treatment, as well as knowing the level of sensitivity to the disease of the main cultivars used in the state. Although we do not yet know the level of economic damage caused by the disease, its occurrence and severity in the last two harvests demonstrate that greater attention is needed to avoid surprises in the future.

Caroline Gulart, Julia Mantelli, Mônica P. Debortoli, Instituto Phytus; Marcelo G. Madalosso, Phytus Institute/Univ. Int. Reg. - Santiago; Leonardo Gulart, IFSC

Article published in issue 189 of Cultivar Grandes Culturas.