For additional information on any of these 159 R 1991-2. 



pests, please refer to: 



Prokopy, R. J., P. J. Powers, D. R. Cooley, and J. 

 W. Gamble. 1991. Peaches, Pears, Plums - Pest 

 Control Guide for Commercial Growers in 

 Southern New England. Univ. of Massachu- 

 setts Cooperative Extension System Circular C- 



Myers, S. C. (ed.) 1989. Peach Production 

 Handbook. Univ. of Georgia Cooperative Exten- 

 sion Service. 



Peach Pests II (appearing in the next issue of 

 Fruit Notes) will discuss foliage and wood feed- 

 ing insect pests. 



Red Stele Resistant Strawberries 

 and Their Role in Epidemiology 

 of Red Stele 



Shahrokh Khanizadeh 



Agriculture Canada St-Jean-sur-Richelieu, Quebec 



Deborah Buszard 



Macdonald College ofMcGill University, Ste-Anne-de-Bellevue y Quebec 



Red Stele of strawberry, caused by 

 Phytophthora fragariae, is one of the most im- 

 portant strawberry diseases. One effective con- 

 trol is to breed red stele-resistant cultivars. 

 Evaluation of strawberry genotypes for suscep- 

 tibility to red stele has been based almost exclu- 

 sively on visual assessment of reddened stele in 

 the root. Visual disease assessments for red 

 stele, however, generally are subjective and 

 frequently are inconsistent. It has been shown 

 that resistant cultivars are not immune to infec- 

 tion, and relatively high numbers of oospores 

 may be observed in the root tissues of inoculated 

 resistant types as well as susceptible genotypes. 

 Therefore, while reddening of the stele in field- 

 grown plants may be used as a preliminary 

 diagnostic feature, visual symptoms should not 

 be used as the sole criterion to assess suscepti- 

 bility. Genotypes in which the pathogen sporu- 



lates to a limited extent in the root may be 

 classified as resistant in genetic studies but it 

 does not imply that they are not infected. 



In breeding trials where infection is evalu- 

 ated, no seedling or cultivar has been found to be 

 completely immune. Thus, the causal organism 

 probably is spread by infected, symptomless 

 roots. Planting stocks should be verified by 

 careful examination of randomly selected roots 

 for the presence of oospores. This relatively 

 simple technique might well be used prior to 

 planting to monitor the health of commercially 

 distributed daughter plants or certified stocks, 

 especially of resistant cultivars. Attention 

 should be paid to the term 'resistant,' which is 

 used widely in horticulture and plant pathology. 

 A cultivar resistant to Phytophthora fragariae 

 may not show any symptoms of red stele even 

 though it is infected by the fungus. 



16 



Fruit Notes, Spring, 1992 



