Agriculture: Rice Pest Management Guidelines

Kernel Smut

  • Tilletia barclayana
  • Symptoms

    Kernel smut appears as a black mass of spores that replaces all or part of the endosperm of individual kernels near or at maturity. Usually, only a small number of kernels on each panicle are infected. Completely smutted kernels may be slightly swollen while others may break open, exposing the dark spores. These black spores make the disease easy to recognize. If disease incidence is high, a dark cloud of spores may be observed coming from combines during harvest.

    Comments on the Disease

    Kernel smut is present in all rice production areas of California. Spores (chlamydospores) liberated during harvest fall to the soil where they overwinter. The fungus may also overwinter in or on seeds. As fields are flooded in the spring, spores float to the surface, germinate, and produce other spores and mycelial stages. At flowering (heading), secondary airborne spores may infect developing ovaries of individual open florets.

    Short and medium grain varieties have lower incidence rates of kernel smut than long grain varieties. Long grain rice varieties may be more susceptible to kernel smut because their florets are open wider and for longer during flowering. Excessive nitrogen fertilization and periods of dew or light rain increase the incidence and severity of kernel smut.

    At low levels, the disease has little to no effect on yield or quality. However, when incidence is high, the disease may cause lower grain yields and reduce milling quality by reducing the whiteness of milled rice.

    Management

    Cultural Control

    Avoid excessive rates of nitrogen fertilizer. Plant short or medium grain varieties in fields with kernel smut history. Plant certified rice seed.

    Organically Acceptable Methods

    All cultural controls discussed above are acceptable in a certified organic crop.

    Monitoring and Treatment Thresholds

    Kernel smut symptoms are not visible until grain starts to mature, around the time fields are drained for harvest. Smutted kernels are more noticeable in the early morning when dew causes the spores to ooze out of the kernel. By the time disease is evident, it is too late for any management action that may reduce its incidence. Because of this, any fungicide application will be preventive and require application prior to expression of visible disease symptoms. Fungicide applications for kernel smut will usually only occur where there is a history of significant disease incidence. The fungicide propiconazole may be effective in reducing the incidence of kernel smut when applied at the mid-boot stage, before panicles have emerged.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. PROPICONAZOLE 6–10 fl oz 24 35
      (Tilt)      
      MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
       
    B. AZOXYSTROBIN/PROPICONAZOLE
      (Quilt Xcel) 14–27 fl oz 12 35
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3)
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with different group numbers are suitable to alternate in a resistance management program. In California, make no more than one application of a fungicide with a mode-of-action group number associated with high resistance risk before rotating to a fungicide with a different mode-of-action group number; for other fungicides, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.
    NA Not applicable
    Text Updated: 07/24
    Treatment Table Updated: 01/24
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