Rice Cercospora Disease – Treating Narrow Brown Leaf Spot Of Rice

By: Tonya Barnett, (Author of FRESHCUTKY)

Sustainability and self-reliance are a common goal among many home gardeners. The quality and benefits of home-grown crops inspire many growers to expand their vegetable patch each season. In this, some are drawn to the idea of growing their own grains. While some grains, like wheat and oats, may grow with ease, many people choose to attempt growing more difficult crops.

Rice, for example, can be grown successfully with careful planning and knowledge. However, many common issues which plague rice plants can lead to reduced yields, and even crop loss. One such disease, narrow brown leaf spot, remains troublesome for many growers.

What is Narrow Brown Leaf Spot of Rice?

Narrow brown leaf spot is a fungal disease which impacts rice plants. Caused by the fungus, Cercospora janseana, leaf spot may be an annual frustration for many. Most commonly, rice with narrow brown leaf spot symptoms manifest in the form of narrow darkened spots on rice plants ranging in size.

Though the presence and severity of infections will vary from one season to the next, well established cases of rice cercospora disease can lead to decreased yields, as well as premature loss of harvests.

Controlling Rice Narrow Brown Leaf Spot

Though commercial growers may have some success with the use of fungicide, it is often not a cost effective option for home gardeners. Additionally, rice varieties which claim resistance to narrow brown leaf spot are not always reliable options, as new strains of the fungus commonly appear and attack plants which demonstrate resistance.

For most, the best course of action as a means to controlling losses related to this fungal disease is to choose varieties that mature earlier in the season. By doing so, growers are able to better avoid intense disease pressure at harvest time late in the growing season.

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This disease is characterized by a wide range of symptoms. However, the presence of circular or oval brown spots with a yellow halo during the tillering stage is the most visible sign of the infection. As they enlarge, a gray center develops in the middle of these spots and a reddish-brown margin becomes visible. Stems also become discolored. On susceptible varieties, lesions may reach a length of 5-14 mm and can cause leaves to wilt. On resistant varieties, the lesions are yellow-brown and pinhead-sized. The infection of florets leads to incomplete or disrupted grain filling and a reduction in grain quality.

The symptoms are caused by the fungus, Cochliobolus miyabeanus. It can survive in seeds for more than four years and spread from plant to plant through airborne spores. Infected plant debris left in the field and weeds are other common ways to spread the disease. Brown spots can occur at all crop stages, but the infection is most critical from maximum tillering to ripening stages. The disease often occurs in fields with mismanagement of soil fertility, mainly in terms of micronutrients. Significant control of brown spot has been achieved using silicon fertilizers. The use of a mixture of cattle manure and chemical fertilizers somewhat also reduces its severity. High humidity (86-100%), prolonged periods of leaf moisture and high temperatures (16-36В°C) are very favorable for the fungi.

Sheath blight (caused by Rhizoctonia solani AG1-IA) and narrow brown leaf spot (NBLS, caused by Cercospora janseana) are among the most important diseases affecting rice production in Texas and in other regions in the southern United States. Strobilurin fungicides have been used extensively to manage these two diseases, especially sheath blight. Unfortunately, heavy reliance on use of fungicides with a single mode of action has induced the development of the strobilurin-resistant isolates of R. solani AG-1 IA in the U.S. NBLS, once considered a minor disease in the U.S, is growing in its occurrence and severity, while little scientific information is available on NBLS management. This created an urgent need for identifying other effective fungicides with different modes of action. A 6-year field study was conducted on rice to evaluate the efficacy of newly registered and unregistered fungicides in comparison to common fungicides for management of sheath blight and NBLS diseases. Single applications of the fungicides containing azoxystrobin, propiconazole, azoxystrobin plus propiconazole, trifloxystrobin plus propiconazole, fluxapyroxad, pyraclostrobin, flutolanil and antibiotics (validamycin and kasugamcyin) were made at the late booting stage. Sheath blight and NBLS severities were rated prior to harvest. All the fungicide treatments were effective in reducing sheath blight severity compared to the untreated control in each year. Propiconazole and fluxapyroxad were more effective in reducing NBLS than other fungicides. Along with reduced sheath blight and NBLS severities, fungicide-treated plots had higher yields than untreated plots.

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