Interpretive Summaries

Alternaria yaliinficiens sp. nov. on Ya Li Pear Fruit: From Interception to Identification. Rodney G. Roberts, United States Department of Agriculture–Agricultural Research Service, Tree Fruit Research Laboratory, Wenatchee, WA 98801. Plant Dis. DOI: 10.1094/PD-89-0134, 2005 (online). Accepted for publication 9 September 2004.

A new species of Alternaria, A. yaliinficiens R. G. Roberts, sp. nov., is described from infected fruit of Chinese Ya Li pear intercepted at U.S. ports of entry in 2001. Strains of the new species were compared with other known Alternaria spp. reported to occur on Ya Li pear fruit (A. alternata and A. gaisen) for the ability to cause disease on Ya Li pear fruit, their microscopic appearance, and several genetic traits. A. yaliinficiens was found to be unique in all of these comparisons, and was formally described as a new species. The disease was named “chocolate spot of Ya Li pear.” The presence of chocolate spot, caused by A. yaliinficiens and other exotic Alternaria spp. on intercepted Ya Li pear fruit, has led to the suspension of the import program for Chinese Ya Li pear each year since 2001.

Comparison of Visual and Multispectral Radiometric Disease Evaluations of Cercospora Leaf Spot of Sugar Beet. K. Steddom, Texas Agricultural Experiment Station, Amarillo, TX 79106; M. W. Bredehoeft, Southern Minnesota Beet Sugar Cooperative, Renville, MN 56284; M. Khan, North Dakota State University and University of Minnesota, Fargo, ND 58105; and C. M. Rush, Texas Agricultural Experiment Station, Amarillo, TX 79106. Plant Dis. DOI: 10.1094/PD-89-0153, 2005 (online). Accepted for publication 15 September 2004.

Visual assessments of disease severity have been criticized for their lack of precision, and their variability among individuals and locations make it difficult to relate results from different studies. Radiometers have been shown to accurately quantify the amount of green leaf tissue in experimental plots and have been recommended for assessment of disease severity. In this article, we show that some radiometric disease assessment methods are more precise than visual assessments and have levels of reproducibility similar to visual assessments. With the exception of one visual disease evaluation, four radiometric methods were better at differentiating treatment differences than visual disease assessments, suggesting a greater level of sensitivity. Where high precision is required or where the potential of bias must be avoided, we recommend using radiometric methods in addition to visual disease assessments.

Characterization of Phytophthora capsici Isolates from Processing Pumpkin in Illinois. S. Z. Islam, M. Babadoost, K. N. Lambert, and A. Ndeme, Department of Crop Sciences, University of Illinois, Urbana 61801; and H. M. Fouly, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana 61801. Plant Dis. DOI: 10.1094/PD-89-0191, 2004 (online). Accepted for publication 26 September 2004.

Illinois ranks first in pumpkin production in the United States, producing about 90% of the country’s commercial processing pumpkins. Phytophthora blight, caused by Phytophthora capsici, has become one of the most serious threats to processing pumpkin production in Illinois and has caused complete crop loss in some fields. P. capsici can attack the host plant at any growth stage, and it causes seedling death, crown rot, foliar blight, and fruit rot. The pathogen also causes severe crop losses in other cucurbits, eggplant, pepper, and tomato. During the past 5 years, we observed differences in the severity of Phytophthora blight and yield losses in the fields, and growth and sporulation of P. capsici in the laboratory. Growers of processing pumpkin reported losses due to seedling death caused by P. capsici. In some pumpkin fields, seedling death was so widespread that the growers had to replant fields two or three times. These reports and observations raised the question of whether the isolates of P. capsici in different processing pumpkin fields vary in virulence. Through this study, the P. capsici isolates examined from processing pumpkin fields in Illinois were demonstrated to belong to six genetic groups and to vary in virulence and morphology. Random amplified polymorphic DNA markers were employed to assess genetic variation among 24 isolates of P. capsici from 10 individual fields at six locations. Inoculation of pumpkin seedlings in the greenhouse revealed that the isolates belonged to six distinct genetic groups differing significantly in virulence. Isolates tested exhibited four growth patterns in culture: cottony, rosaceous, petaloid, and stellate. P. capsici isolates with cottony growth pattern did not grow at 36°C. Nine of 24 isolates tested produced chlamydospores (thick-walled spores), which has not been reported for P. capsici from pumpkin. Chlamydospores enable the pathogen to survive over winter in the field.

Use of a Rainfall Frequency Threshold to Adjust a Degree-Day Model of Ascospore Maturity of Venturia inaequalis. Arne Stensvand and Håvard Eikemo, Norwegian Crop Research Institute, Plant Protection Centre, Department of Plant Pathology, N-1432 Ås, Norway; and David M. Gadoury and Robert C. Seem, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. DOI: 10.1094/PD-89-0198, 2005 (online). Accepted for publication 8 October 2004.

Apple scab is a destructive fruit disease that can be controlled in commercial apple production only through the use of fungicides. A key factor driving the timing and intensity of spraying is the supply of spores of the pathogen, which overwinters in leaf litter on the orchard floor. The spores are discharged into the air during rain, and the supply is eventually exhausted after several rain events. Knowing exactly when the supply of spores is exhausted is of great interest, because this signals the switch to a less-intensive use of fungicides. A simple model, driven by degree-day accumulation, was developed in 1982 for this purpose. Under conditions of frequent rain, the model works quite well. However, what about years with extended dry periods? Our research shows that “turning off” degree-day accumulation on the seventh consecutive day without rain greatly enhances the accuracy of the mode in dry years, without disrupting accuracy in wet years. The use of the above “rainfall frequency threshold” is a simple modification that will make the model more flexible, and adaptable to drier climates.