**1. Introduction**

Rice originated 130 million years ago to become the annual cereal crop that provides 20% of the essential calories needed to feed more than one-half of the world population [1]. Major producing countries of rice are China and India; most of the rice produced by both countries is domestically consumed. In the USA, rice has been under cultivation for over three hundred years; however, today less than 2 percent of world rice is being produced in the USA. The rice crop in the USA is known for its high rough rice yield, and excellent milling and cooking quality that occupies the top 10 in the international marketplace. The earliest rice seeds were discovered over 7,700 years ago in the Hangzhou area, eastern China [2] establishing China as the first ancient civilized country to grow rice. The domestication and agronomic improvement of rice began with the exploitation of wild rice and land race varieties during ancient times. Since then, the primary breeding objective has been to increase the yield potential to meet the rapidly increasing demands of human consumption. In the 1950s, the semi-dwarf gene *sd1* was discovered and used to develop semi-dwarf varieties with high grain yield but without lodging. The rapid utilization of this technology began the green revolution in rice. In 1960, the principles and techniques of hybrid production developed by American corn

breeders were adapted for hybrid rice breeding in China [3]. During the 1970s, indica types of hybrid rice were rapidly deployed in major rice producing areas and, currently, hybrid rice is more than 50% of the rice production in China. Hybrid rice production has expanded to other countries and in 2020, occupied approximately 35% of US rice acreage. Globally rice crops have been well protected against diseases; however, throughout this intense agronomic selection for yield enhancing genes, the corresponding genetic diversity needed for effective rice disease control has decreased [4]. Increasing yield through hybrid rice is one way to increase the total rice production. However, rapid extension of hybrid rice worldwide will present a new challenge for the control of rice diseases such as rice blast and sheath blight diseases since limited germplasm can be used for hybrid seed production. An insignificant race of the southern leaf blight fungus *Bipolar maydis* under favorable conditions resulted in 6 billion crop loss when maize hybrids with cytoplasm (cms-T) were heavily deployed in the southern USA (for example, [5]). Understanding mechanisms of interactions of rice with harmful microbes are therefore critical for food security.

Most diseases of rice are caused by harmful fungi such as blast disease caused by [*Magnaporthe oryzae* (anamorph: *Pyriculara oryzae*)], sheath blight [*Rhizoctonia solani* (telomorph: *Thanatephorus cucumeris*)], brown spot [*Cochliobolus miyabeanus* (anamorph: *Bipolaris oryzae*)], false smut (*Ustilaginoidea virens*), kernel smut [*Tilletia barclayana* (*Neovossia horrida*)], Narrow brown leaf spot [*Cercospora janseana* (telomorph: *Sphaerulina oryzina*)], crown sheath rot (*Gaeumannomyces graminis* var. *graminis*), downy mildew (*Phytophthora macrospora*), aggregate sheath spot [*Rhizoctonia oryzae-sativae* (Sawada) Mordue and *R. oryzae* Ryker & Gooch)], eyespot [*Drechslera gigantea* (Heald et Wolf) S. Ito], leaf smut (*Entyloma oryzae*), leaf scald (*Microdochium oryzae*), seedling blight (*Pythium*, *Fusarium*, *Diplodia*, *Rhizoctonia*, and *Penicillium* spp), stem rot (*Phytophthora sojae*), bakanae [*Fusarium moniliforme* (syn. F. verticilloides), teleomorph*: Gibberella fujikuroi* (syn. *Gibberella moniformis*)], respectively. The next most common diseases are caused by harmful bacteria such as leaf blight caused by *Xanthomonas oryzae* pv. *oryzae* (*X. campestris* pv. *oryzae*), bacterial panicle blight [*Burkholderia glumae* and *Burkholderia gladioli* (Severin)], bacterial leaf streak (*Xanthomonas oryzae* pv. *Oryzicola*), bacterial foot rot (*Dickeya zeae*); sheath brown rot (*Pseudomonas fuscovaginae*), bakanae [*Fusarium moniliforme* (syn. *F. verticilloides*), teleomorph: *Gibberella fujikuroi* (syn. *Gibberella moniformis*)], respectively [6].

Any of the above mentioned rice diseases can result in severe yield loss when ideal circumstances favor disease infection and development. Among them, blast is the most devastating rice disease worldwide [6–8]. Disease symptoms of blast are more pronounced on rice under high nitrogen based nutrients and drought stress conditions, and often seen on rice plants grown on levees and the edges of rice paddies. Blast disease annually significantly reduces the crop in upland and often in flood irrigated rice production as well. Three notable crop damages are 1) the widespread destruction of the cultivar 'Newbonnet' in 1980s in the US; 2) 45% of rice affected by blast in 1993 in Japan; and 3) blast disease caused significant damage in 106 million hectares from 1982 to 2005 in China [9, 10]. Presently, occurrence and severity of blast are becoming more widespread in the Southern US and California. Sheath blight causes damage on all rice cultivars, especially on semidwarfs [11]. Sheath blight disease is the second most damaging disease after blast worldwide. Sheath blight was considered a minor disease for many years but has become more destructive under intensified high input production systems. The damage due to sheath blight was estimated to be from 20–42% in a simulation study [12] and 50% of crop loss under favorable conditions in the USA [13]. To date, sheath blight reduces the crop more than that of rice blast in the USA.

*Physiological, Ecological and Genetic Interactions of Rice with Harmful Microbes DOI: http://dx.doi.org/10.5772/intechopen.97159*

Understanding impacts of rice exposed to the above mentioned harmful microbes under changing climates is a never ending challenge. Rapidly evolving technologies such as genome sequencing, computational biology and genome editing methods have been used to study how nucleotide sequence changes influence the outcomes of host-pathogen interactions. The aim of this chapter is to update the complex interactions between rice plants with the harmful microbes causing diseases. Emphasis is placed on some aspects of the physiological, ecological, and genetic interactions between the rice plant and harmful pathogens such as rice blast (*M. oryzae*) and sheath blight (*R. solani*).
