**1.2 Sheath blight disease**

*Protecting Rice Grains in the Post-Genomic Era*

**1.1 Rice blast disease**

and pesticides at its disposal. Yield per hectare is currently about 7.5 tons/hectare [1] and has been one of the top breeding priorities. Rice breeding programs in the USA are associated with private companies such as Rice Tech Inc., BASF, and major state university agriculture experiment stations, consisting of one or more rice breeders, pathologists, and other scientists. Additionally, the USDA Agriculture Research Service (ARS) has conducted research in Stuttgart, Arkansas, since 1931 [2]. Soon after the establishment of the USDA, ARS, Dale Bumpers National Rice Research Center (DB NRRC, 1998), the molecular plant pathology program has been performing translational research to tackle the major constraints of rice production.

One of the major constraints for rice production in the USA is rice blast. Blast disease of rice is caused by the filamentous fungus *Magnaporthe oryzae*

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**Figure 1.**

*Photographs showing symptoms of leaf and panicle blast and asexual spores of rice blast fungus. (A) Panicle damage caused by blast; (B) severe blast lesions on rice seedlings affecting rice seedling establishment; (C) blast lesions on a rice leaf after diseased leaf from a field was placed in a petri dish with a prewetted filter paper for 24 h; and (D) four asexual spores of the rice blast fungus [9]. The pictures were taken either with an iPhone,* 

*with a dissecting microscope, or with a Nikon eclipse microscope.*

The soil-borne, necrotrophic *Rhizoctonia solani* species have a wide range of host plant species. The anastomosis group AG1-IA of *R. solani* infects rice and causes sheath blight disease. *R. solani* is a monocyclic fungus. The life cycle of *R. solani* begins with mycelia growth from sclerotia soon after attachment onto rice seedlings/plants. The mycelia then move upward along the sheaths and leaves of rice plants, ultimately resulting in damages on the sheaths, leaves, and grains. The life cycle ends with the formation of overwintering structures, sclerotia on the sheaths, leaves, seeds, and in soils [10] (**Figure 2**).

### **Figure 2.**

*Photographs showing sheath blight disease on the sheaths, leaves, and grains (A and B) and young mycelia sheath blight fungus with 45 and 90° angles (C). Pictures were taken with an iPhone or with a Nikon eclipse microscope.*

### **1.3 The epidemics, climate, and damages**

In the Southern US, rice blast disease can be found annually and occasionally results in significant crop damages. However, sheath blight disease occurs more often than blast disease partially due to high-density cultivation. An extended dew period and light are known to stimulate sporulation of *M. oryzae*. Light rain is known to keep plant surfaces wet and create near 100% relative humidity, helping the attachment and penetration of the conidia of *M. oryzae*. High humid conditions also favor the growth, infection, and spread of *R. solani* to other leaves and other plants [10]. In California, there is no rain during the rice-growing season. As a result, significant yield loss due to blast has not been reported [11]. Sheath blight disease has not been reported in California either despite a phenotypically similar disease, the aggregate sheath spot of rice caused by *Rhizoctonia oryzae-sativae* [12], commonly occurring. Presently, substantial fungicides have been used to prevent crop losses of these fungal diseases in the USA.
