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= Brown Spot Disease Of Rice =

Symptoms

 * Brown Spot Disease of rice normally affects the leaves and glumes of the rice plant . However, the seedlings, sheaths, stems, and grains of adult host plants can be affected as well. The major symptom typically manifests itself as evenly spaced oval-shaped lesions around 1cm in length . When fully developed the spots are brown with greyish centers. Infections that form in unfavorable conditions appear as small dark-brown spots whereas those formed in favorable conditions appear as black/dark brown lesions with a velvety aspect due to impending sporulation. Other symptoms to look for are abnormal coloring of the leaves and stems, lesions on seeds, and fungal growth on the leaves.

Disease Cycle

 * Starting with the “dormant phase” of the Brown Spot disease cycle. The fungus Cochliobolus miyabeanus survives winters/time between growing seasons as conidia and mycelium within seeds, debris, volunteer rice, and weeds . In the spring when the rice seeds are planted the pathogen enters its “active phase” . Here the pathogen initiates a local infection of all parts of the host. As the host grows, so does the pathogen, eventually developing conidiophores & conidia through the stomata of the host plant. These conida are then disseminated via wind, rain, etc. They can be spread to other healthy plants where they will infect the inflorescence of the rice plant or create new infection on the already diseased host . Either way, the proceeding infection has created extensive amounts inoculum (conidia, mycelia) to overwinter and repeat the process in the following growing season.

Management

 * Brown spot disease on rice can be effectively managed in a number of ways. The first step to managing Brown Spot disease is to perform cultural control and improve soil fertility. This can be done by monitoring soil nutrients regularly and applying required fertilizers when necessary. However, fertilizers can be very costly and take several cropping seasons for them to become effective . In this case, more economical approaches can be taken to treat/prevent Brown Spot disease.The most economical way to manage this disease is the development of host resistance through breeding . Of the 25 varieties of rice screened for resistance to Brown spot disease in 2014/2015: four were highly resistant,  seven were resistant, six were moderately resistant, three were moderately susceptible, and three were highly susceptible . If resistant varieties are unavailable, fungicides such as iprodione, propiconazole, azoxystrobin, trifloxystrobin, and carbendazim are an effective alternative when applied as seed treatments . If neither of these are available, soaking the seeds in hot water (53°C-54°C) for 10-12 minutes prior to planting can help control primary infection at the seed stage . All of these methods of control, except for fungicides (with exceptions) and resistant varieties, aim to stop the pathogen while in its dormant state. Fungicides, which can be applied to seeds in order to stop the disease while in its dormant state may also be used as a foliar application to prevent the germination of conidia on mature plants. Resistant cultivars, while able to deter infection at the seed stage also continue to deter infection throughout the life stages that follow.

Pathogenesis

 * When infected with Cochliobolus miyabeanus, the causal agent of Brown Spot disease, rice plants undergo a series of changes in hormone levels and gene expression to combat infection . These levels of response vary depending on the level of resistance to the fungal pathogen. When a resistant cultivar is inoculated with the pathogen a hypersensitive response is activated within the host cells . This leads to microscopic cell death in the infected areas to combat the spread of infection within the plant. When a susceptible cultivar is inoculated with the pathogen, no hypersensitive response is activated. However, comparable levels of microscopic cell death can be observed but without the blockage of fungal ramifications. Expression of PR and JAmyb genes are statistically identical in both susceptible and resistant hosts .This means that while both resistant and susceptible hosts may have equal responses of PR and JAmyb gene (plant defense responses to disease), susceptible hosts are unable to quarantine the infection and stop the spread of disease. These responses are specific to the host and Cochliobolus miyabeanus. While other rice pathogens evoke similar responses within the host they are statistically different from each other, showcasing the variability of effector responses within rice plants.