**Abstract**

Rice blast, caused by the fungus *Magnaporthe oryzae* (anamorph: *Pyricularia oryzae*), is a ubiquitous disease that threatens rice production in the USA and worldwide. Growing resistant cultivars is the most economical and effective way to manage this disease. Multiple races exist in the *M. oryzae* population in the USA. It is necessary to know the resistance spectrum of rice cultivars to the prevalent rice blast races in the areas where they are grown. Twelve isolates of *M. oryzae* collected from the southern US rice-growing region were used in this study. The genetic diversity of these isolates was evaluated with genetic and molecular methods, and the pathogenicity to different rice blast resistance genes was determined by the disease reaction of two sets of near-isogenic lines containing one blast *R* gene per line. From 2005 to 2016, about 200 Uniform Regional Rice Nursery (URRN) breeding lines have been tested with 9–12 reference isolates annually, and a total of 2377 breeding lines have been tested. The varieties with good resistance to rice blast disease have been identified. The results could be useful for the management of rice blast disease in the southern US rice production area.

**Keywords:** rice, blast disease, avirulence gene, resistance gene, breeding lines

### **1. Introduction**

Rice is one of the most important staple food crops worldwide, feeding over half of the world's population [1]. The demand for rice continues to increase with the increase in the global population. The USA grows approximately 1.5 million hectares of rice annually and produces about 8–11 million metric tons of rice valued at 3.6 billion dollars (**Figure 1**) [2] . Although the USA is a relatively small rice producer accounting for less than 2% of the total rice production worldwide, it is a major rice exporter that occupies 6–13%, with an average of 10%, of the world rice export market (**Figure 1**), making the USA one of the top rice exporters in the world [3].

Rice blast disease, caused by the fungus *Magnaporthe oryzae* (anamorph: *Pyricularia oryzae*), is one of the most important diseases on rice worldwide and is responsible for approximately 30% of rice production losses globally [1, 4]. A wide range of management practices have been used to reduce losses from rice blast. For example, cultural practices such as crop rotation, controlling the timing and amount of nitrogen applied, and managing the flood depth in the field may reduce the impact of blast [5]. A number of fungicides also are effective in managing rice

**Figure 1.**

*Rice production in the USA and its percentages of world total rice production and export. Mha, million hectare; MMT, million metric tons.*

blast disease [4]; however, it is not a preferred management option due to environmental concerns and cost. Growing resistant cultivars is the most economical and effective way to manage this disease [4, 6]. Many rice blast *R* genes have been characterized, some of which have been widely used in rice breeding programs worldwide [6–8]. The *R* genes recognize the corresponding specific avirulence genes from the pathogen and initiate defense mechanism [9]. For example, the *R* gene *Pita* can interact with the counterpart *AVR-Pita* from the pathogen and confer resistance [10]. However, the changes in avirulence genes can result in the loss of function of the corresponding *R* genes. For example, the *R* gene *Pita* has deployed in rice cultivars in the southern USA and provided durable resistance for a long period of time [11], but the resistance of the *Pita* gene was overcome by race IE1k in 2004 [12].

The population of *M. oryzae* in the southern USA has been intensively studied [13–18]. Multiple races exist in the *M. oryzae* population in the USA. For example, race IB49 and IC17 were the most prevalent races in Arkansas [13–15], with occasional epidemics due to race IE1k or "race K" type isolates [12]. Near-isogenic lines, each containing a targeted blast resistant gene, in either a Japonica-type variety Lijiangxingtuanheigu (LTH) background [19] or Indica-type CO39 background [20], have been used for race identification in Asia [21]. In the USA, the *M. oryzae* population has been intensively studied [13–18, 22], but the relationship between races to individual rice blast *R* genes in the USA is largely unknown [22]. In addition, it is necessary to evaluate the resistance spectrum of newly developed rice breeding lines to the prevalent rice blast races in the southern US rice-growing region before they are released.

The objective of this study was to summarize the disease reactions of a wide range of rice germplasm from the Uniform Regional Rice Nursery (URRN) lines to 12 reference isolates of the rice blast pathogen from 2005 to 2016.

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**Table 1.**

*Evaluation of Resistance of US Rice Breeding Lines to the Rice Blast Pathogen*

**2. Diversities of the 12 US reference isolates of** *M. oryzae*

Twelve isolates of *M. oryzae*, collected from the southern USA, were used as reference isolates to test the URRN lines during 2005–2016 (**Table 1**). Among them, six isolates (49D, #24, A119, A264, A598, IB33) were collected from AR; four isolates (TM2, ID13, ZN7, and ZN15) were collected from TX; one isolate, IB54, from LA; and one isolate, ZN46, from FL (**Table 1**). These isolates represented 10 races, including IB49 (49D, A119, and A598), IB33, IB54, IC1 (ZN46), IC17 (A264), ID13, IE1 (ZN7), IE1k (TM2), and IG1 (#24). Most isolates were used each year on a different set of URRN lines. Isolate IB33 has been tested in 11 years but not in 2007. Isolate IB54 has not been tested until 2009. Isolate ID13 has been tested in 7 years,

The genetic diversity of the 12 reference isolates was evaluated by vegetative compatibility analysis [13] and molecular methods. Vegetative compatibility analysis indicated that three isolates A598, ZN15, and ZN46 belonged to vegetative compatibility group (VCG) US-01; isolates TM2, #24, and A264 belonged to VCG US-02; two isolates 49D and A119 belonged to VCG US-03; and other two isolates IB33 and IB54 belonged to VCG US-04 (**Table 1**). The VCG of isolate ID13 was not

Using Pot 2 primers [23], the repetitive element-based polymerase chain reaction

(Rep-PCR) was used to DNA fingerprint the 12 reference isolates. The amplicon patterns of 49D, IB33, and IB54 based on Pot 2 primers were identical; TM2 and ZN7 were identical to each other; isolate 24 and A264 were identical to each other, but they had one extra band compared to that of TM2 and ZN7; ZN15 and ZN46 had similar patterns (**Figure 2**). The mating types of these isolates were determined by using mating-type-specific primers [24]. The results suggested that six isolates, 49D,

**Isolate Vegetative compatibility group (VCG) Mating type RACE Year Origin** 49D US-03 I IB49 1985 AR TM2 US-02 II IE1K 2004 TX #24 US-02 II IG1 1992 AR A119 US-03 II IB49 1992 AR A264 US-02 II IC17 1993 AR A598 US-01 I IB49 1992 AR IB33 US-04 I IB33 AR IB54 US-04 I IB54 1959 LA ID13 II ID13 1982 TX ZN7 US-02 II IE1 1995 TX ZN15 US-01 I IB1 1996 TX ZN46 US-01 I IC1 1996 FL

*Background information on the 12 US reference isolates of M. oryzae used in this study.*

*DOI: http://dx.doi.org/10.5772/intechopen.84980*

**2.1 Twelve US reference isolates of** *M. oryzae*

but not in 2005, 2007, 2009, 2013, and 2014.

determined.

**2.2 Genetic diversity of the 12 reference isolates**
