33. Rice Blast and Their Losses

Introduction of Rice:

    Oryza sativa, commonly known as rice, is a crucial staple crop worldwide, particularly in Asia, where it provides a significant source of dietary carbohydrates and calories for roughly half of the global population. In addition to its nutritional value, rice serves as a vital economic crop in many countries, contributing to foreign income through exports. Each year, billions of tons of rice are consumed globally, with billions more being exported to various regions around the world.

• Rice is a grain that comes from either the Oryza sativa (Asian rice) or Oryza glaberrima (African rice) species. 
• There are more than 40,000 different varieties of rice, with Basmati, Thai Jasmine, and Italian Arborio being among the most popular.
• Interestingly, two Japanese car brands were named after rice: Toyota, which translates to "Bountiful Rice Field," and Honda, which means "Main Rice Field."
• Growing rice requires a significant amount of water, with approximately 5,000 liters needed to produce just one kilogram of rice.

1. Rice Blast:

Causal Organism:

    Pyricularia oryzae

Hosts and Symptoms:

    M. grisea is a pathogenic ascomycete fungus that can reproduce both sexually and asexually to produce specialized infectious structures called appressoria. These structures can infect aerial tissues, while hyphae can infect root tissues, making the fungus highly effective as a plant pathogen.

    Rice blast disease has been reported in various rice strains, including M-201, M-202, M-204, M-205, M-103, M-104, S-102, L-204, and Calmochi-101. Among these, M-201 is the most susceptible to infection.

• Leaf Blast: The crop is susceptible to fungal attacks throughout all stages of its growth, with visible signs of infection appearing on its leaves, nodes, rachis, and glumes. Lesions may start as tiny, bluish-green flecks on the leaves, which can expand during periods of high humidity to form spindle-shaped spots with a grey center and a dark brown margin. This symptom is commonly known as "Leaf Blast."

• 
  Nodal Blast: As the disease progresses, the spots on the leaves will merge, resulting in the drying and withering of large areas of the affected leaves. In addition to the leaves, spots may also develop on the sheath, and severe infections can cause the entire nursery or field to appear burnt. The fungus may also cause the formation of black lesions on the nodes, which can girdle and weaken them. This can result in the breakage of the affected nodes and death of the plant parts located above them, a symptom referred to as "nodal blast."
• Neck Blast: As the crop enters the flowering stage, the fungus can also infect the peduncle, causing the appearance of brownish-black lesions. This symptom is commonly referred to as "rotten neck," "neck rot," or "panicle blast" and is sometimes also called "neck blast."

    Early neck infection can result in the failure of grain filling, whereas late disease may only allow partial grain filling to occur. Additionally, small brown to black spots may be visible on the glumes of heavily infected panicles. The fungus can significantly reduce crop yields, with losses ranging from 30 to 61 percent depending on the stage of infection.

Pathogen:

• The mycelium of the fungus is typically hyaline to olivaceous in color and contains multiple septa. 
• Conidia, pyriform to ellipsoid in shape, are produced in clusters on long, septate, olivaceous conidiophores. These conidia have a broad base with a hilum and are typically 3-celled and hyaline to pale olive green in color. 
• The perfect state of the fungus, M. grisea, produces perithecia. The ascospores are hyaline, slightly curved, fusiform, and typically have 4 cells.

Favorable Conditions:

• Rice blast disease is favored by intermittent drizzles, cloudy weather, more rainy days, longer periods of dew, and high relative humidity ranging from 93-99 percent. 
• Additionally, low night temperatures below 26˚C or between 15-20˚C, the availability of collateral hosts, and excessive nitrogen can contribute to the development and spread of the disease. 
• To forecast rice blast disease, a combination of factors must be considered, including a minimum night temperature range of 20-26˚C, a high relative humidity of 90 percent or above, and these conditions lasting for at least one week during any of the three susceptible phases of crop growth: the seedling stage, post-transplanting tillering stage, and neck emergence stage. 
• The first leaf blast forecasting model in Japan was developed and known as BLAST.

Disease Cycle:

    Rice blast disease is primarily spread through airborne conidia, which are present throughout the year. The fungus can also be transmitted through infected straw and seeds, and irrigation water can carry conidia to different fields. Collateral hosts such as Panicum repens, Digitaria marginata, Brachiaria mutica, Leersia hexandra, and Echinochloa crusgalli can also serve as a source of survival for the fungus.

    When spores land on rice leaves, they can germinate, penetrate the leaf, and cause a lesion within four days. The fungus can produce more spores in as little as six days. Infections that result from spores arriving from a distance are known as primary infections, which typically result in a few scattered spots on leaves. However, spores that arise from primary infections can lead to many more infections, a process known as the secondary spread. Secondary spread is responsible for the severe epidemics of rice blast disease in fields and localized areas.

Management:

• To prevent disease in crops, it's recommended to cultivate moderately resistant varieties such as CO47, IR 20, ADT36, ADT39, ASD 18, and IR64, while avoiding highly susceptible varieties like IR50 and TKM6 during disease-prone seasons. 
• It's also important to remove weeds from the field bunds and channels.
• Treating seeds with Captan, Thiram, Carbendazim, Tricyclazole, or Pseudomonas fluorescens at a rate of 2 g/kg can help prevent disease. In the nursery, spraying with Carbendazim (500mg/L) or Tricyclazole (300mg/L) is recommended.
• To protect the main field, it's advisable to spray with Edifenphos (500 ml), Carbendazim (500 g), Tricyclazole (500 g), or Iprobenphos (IBP) (500 ml/ha).

Global Losses:

• Scenario three analysis showed that an average of $69.34 million is lost annually to blasts in the Mid-South of the US, due to yield loss and mitigation costs.
• If the maximum infection rate of 46.95% is applied to susceptible hectares, the potential economic loss is estimated to be $203.49 million annually.
• The calculated potential economic loss as a share of the total value of rice production in Mississippi, Louisiana, and Arkansas is estimated at 3.98%, 4.02%, and 4.92%, respectively.
• The total value of rice production data was obtained from USDA and converted to 2014 dollars using annual CPI retrieved from IMF.
• Overall, the potential economic loss for scenario three is estimated at 4.08% as a share of the total value of rice production in the Mid-South of the US.
• The rice blast caused by Magnaporthe oryzae is a significant threat to global food security, resulting in around 30% of rice production losses worldwide.
• These losses have a direct impact on consumer welfare and increase the global price of rice, which can adversely affect food security.

References:

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