Copes Warren E, Scherm Harald
USDA-ARS Thad Cochran Southern Horticulture Laboratory, P.O. Box 287, Poplarville, MS 39470.
Department of Plant Pathology, University of Georgia, Athens, GA 30602.
Plant Dis. 2010 Jul;94(7):891-897. doi: 10.1094/PDIS-94-7-0891.
Rhizoctonia web blight, caused by binucleate Rhizoctonia spp., is an annual problem in the southern United States on container-grown azaleas (Rhododendron spp.) that receive daily irrigation. Disease progress was assessed weekly from mid-May to mid-September on nursery-grown plants at three locations in Mississippi and Alabama in 2006, 2007, and 2008. Disease onset, defined as the appearance of blighted leaves at the exterior canopy of at least one plant, occurred on average on 20 July, and calendar date was a more precise predictor of disease onset than several combined time-weather variables. Disease progress curves exhibited weekly fluctuations around a typically exponential increase in the mean number of symptomatic leaves per plant until early to mid-September, after which web blight severity leveled off or declined due to disease-induced leaf dehiscence and the appearance of new, asymptomatic leaves. Based on the relative increase in the log-transformed number of infected leaves per plant, weekly assessment periods were classified as having slow (≤0%), intermediate (>0 to <10%), or rapid (≥10% increase) disease progress. Three-day moving averages (MA) of various weather variables were calculated, and lagged values (by 5 days) of the MA were used in an attempt to predict disease progress as slow, intermediate, or rapid. Of the periods assessed as having slow disease progress in the 2006-2007 data set (model development data), 90.6% (29 of 32) met at least one of the following heuristically derived criteria for the lagged MA: min. temperature < 20.0°C, max. temperature > 35.0°C, avg. vapor pressure deficit < 2.50 hPa, or day of the year > 240 (28 August). One or more of these same criteria were met in 5 of 16 (31.2%) assessment periods with rapid disease progress, indicating that periods with slow versus rapid disease progression could be distinguished reasonably well based on weather. Results were similar for the 2008 validation data. However, weather variables were not useful in separating periods with either slow or rapid disease progress from those having intermediate progress. Instead, weather variables were most useful when used in a negative-prognosis approach to predict disease progression as being "not rapid" (which includes slow and intermediate periods) or "not slow" (including intermediate and rapid periods). The data set was further analyzed using Classification and Regression Tree (CART) analysis to relate weekly disease progress periods to weather variables. The resulting CART model agreed with the heuristic approach in that temperature variables were more prominent than moisture variables in classifying disease progress periods. With both approaches, satisfactory accuracy was accomplished only with negative-prognoses that classified disease progress periods as not rapid or not slow based on temperature and moisture limits.
由双核丝核菌引起的丝核菌叶网枯病，在美国南部是容器栽培杜鹃花（杜鹃属植物）每年都会遇到的问题，这些杜鹃花需要每日灌溉。2006年、2007年和2008年，于5月中旬至9月中旬每周对密西西比州和阿拉巴马州三个地点苗圃种植的植株进行病害进展评估。病害始发被定义为至少一株植株外部冠层出现枯萎叶片，平均发生在7月20日，与几个综合的时间 - 天气变量相比，日历日期是病害始发更精确的预测指标。病害进展曲线在9月上旬至中旬之前，每株有症状叶片平均数呈典型指数增长的同时呈现每周波动，之后由于病害导致叶片脱落以及新的无症状叶片出现，叶网枯严重程度趋于平稳或下降。根据每株感染叶片对数转换后的相对增加量，每周评估期被分类为病害进展缓慢（≤0%）、中等（>0至<10%）或快速（≥10%增长）。计算了各种天气变量的三日移动平均值（MA），并使用MA的滞后值（滞后5天）来尝试预测病害进展为缓慢、中等或快速。在2006 - 2007数据集（模型开发数据）中被评估为病害进展缓慢的时期，90.6%（32个中的29个）至少符合以下基于滞后MA启发式得出的标准之一：最低温度<20.0°C、最高温度>35.0°C、平均水汽压差<2.50 hPa或一年中的天数>240（8月28日）。在16个病害进展快速的评估期中有5个（31.2%）符合这些相同标准中的一项或多项，这表明基于天气情况可以较好地区分病害进展缓慢与快速的时期。2008年验证数据的结果类似。然而，天气变量对于区分病害进展缓慢或快速的时期与中等进展的时期并无帮助。相反，当以负面预后方法使用天气变量来预测病害进展为“不快速”（包括缓慢和中等时期）或“不缓慢”（包括中等和快速时期）时，天气变量最为有用。使用分类与回归树（CART）分析对数据集进行进一步分析，以将每周病害进展时期与天气变量相关联。所得的CART模型与启发式方法一致，即在对病害进展时期进行分类时，温度变量比湿度变量更为突出。两种方法都只有通过基于温度和湿度限制将病害进展时期分类为不快速或不缓慢的负面预后才能实现令人满意的准确性。
