Evaluation of Methods to Detect Fruit Infected by Monilinia vaccinii-corymbosi in Mechanically Harvested Rabbiteye Blueberry.

Abstract

Blueberry fruit infected by Monilinia vaccinii-corymbosi (causal agent of mummy berry disease) are unfit for processing because of the formation of hardened structures (pseudosclerotia) within them. In commercial packinghouses in Georgia, fruit loads exceeding the tolerance level for mummy berry are appraised at lower quality grades, resulting in severe economic penalties to producers. Two methods to detect and enumerate mummy berry in blueberry loads were evaluated in the laboratory using fruit samples with known numbers of infected fruit. The first method involved destructive processing of the samples in a blender. The resulting blueberry puree was passed through a series of screens and the number of pseudosclerotia of M. vaccinii-corymbosi retained on the screens assessed tactilely. The second method consisted of visual symptom assessment of intact fruit. Bias and coefficients of variation of the blender method in five experiments ranged from -63.0 to 152.4% and 6.9 to 44.1%, respectively, indicating that the method was inaccurate and imprecise. Several factors probably contributed to its poor performance, including the formation of multiple fragments from single pseudosclerotia during blending and subjectivity in the tactile assessment of pseudosclerotia. Bias and coefficients of variation of the visual assessment method in four experiments ranged from -3.41 to 1.97% and 1.16 to 5.17%, respectively. Thus, the visual method was considerably more accurate and more precise than the blender method. Visual assessment was further evaluated under commercial packinghouse conditions, with >70,000 fruit assessed individually for symptoms of mummy berry and other abnormalities. Bias ranged from -11.1 to 33.3%, indicating that visual assessment was less accurate under packinghouse conditions than under laboratory conditions. This was due to the low number of infected fruit encountered in most of the loads, which resulted in large relative errors if only a single fruit was misidentified. In a two-year packinghouse survey, a high incidence of partial infection, together with successional variations in discoloration of infected portions of the fruit as the harvest season progressed, resulted in a greater variation of mummy berry symptoms than previously described.