In rice, black spot damage (or pecky rice; Fig.1) is one of the most important quality factors for grading, marketing, and end-use value (Wang et al., 2002). The pecky rice rice is characterized as a roughly circular lesion, which in some cases appears as a shrunken area. In general, this kind of defect is accompanied by a brown to black grain discoloration of the whole kernel or portion of the kernel. This complex disorder involves many fungi, the white-tip nematode and insect damage. High winds at the early heading stage may cause similar symptoms. Heading applications of stink bug insecticides and fungicides targeted at other more important diseases will reduce the peck rice occurrence. Black spot causes both yield and quality losses that affect rice end-use. Pecky grains weigh substantially less than normal grains since they are not fully developed or are damaged by fungus which results in a yield loss.

Figure 10. Rice grains affected by black spot damage

The new developed model calculates the percentage of rice kernels affected by black spot damage as a function of average temperature and relative humidity excursion during the first 20 days after heading. In particular the output is achieved by multiplying the maximum percentage of pecky grains that is usually expected at the end of the season for the simulated rice variety for three factors, related to temperature and relative air humidity perceived by the crop during the ripening period.

The main equation is given below:

Where:

BS (%) is the percentage of rice grains affected by black spot damage;

DT20 (unitless) is the factor referring to the average daily thermal excursion perceived by the crop during the first twenty days after heading;

DHH20 (d) is the factor referring to the number of days with high air humidity during the first 20 days after heading;

DHTE20 (mm) is the factor related to the number of days with high daily thermal excursion during the first 20 days after heading;

BSavg (%) is the maximum percentage of pecky grains that are usually expected at the end of the season for the simulated rice variety.

The three agro-meteorological factors are computed as follow:

Where:

ΔT20 (°C) represents the average daily thermal excursion perceived by the crop during the first twenty days after heading;

ai (0.1823 for Loto, 0.1096 for Gladio; unitless), bi (-5.0672 for Loto, -3.7312 for Gladio; unitless), ci (43.7063 for Loto, 32.8166 for Gladio; unitless) are coefficients of the equation.

Where:

dH20 (d) are the days with high air humidity, i.e. the number of days characterized by daily humidity excursion lower than a critical threshold (default value equal to 50% ) during the first 20 days after heading;

ai (0.0433 for Loto, 0.0135 for Gladio; unitless), bi (-1.006 for Loto, -0.2914 for Gladio; unitless), ci (9.0008 for Loto, 4.5034 for Gladio; unitless) are coefficients of the equation.

Where:

HΔT20 (°C) are the days with high daily thermal excursion, i.e.  the number of days characterized by daily thermal excursion higher than a critical threshold (default value equal to 10 °C ) during the first 20 days after heading;

hi (0.0079 for Loto, 0.0165 for Gladio; unitless), li (-0.0002 for Loto, -0.0044 for Gladio; unitless), mi (3.9419 for Loto, 2.2072 for Gladio; unitless) are coefficients of the equation.

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