Fruit-Rot Resistance In the American Cranberry, Vaccinium Macrocarpon

Cranberry fruit rot poses a serious economic threat to US cranberry growers and is increasing in severity and scope. In New Jersey, crop losses of 25% are common even with a full fungicide regimen. Fruit rot is caused by a dynamic complex of 10-15 fungal species; primarily Phyllosticta vaccinii, Physalospora vaccinii, Colletotrichum gloeosporioides, Coleophoma empetri and Phomopsis vaccinii. We evaluated our cranberry germplasm collection to identify potential sources of field fruit rot resistance. Fungicides were withheld in 2003 and 2004 to provide extreme disease pressure, and accessions were rated for fruit rot. Seventy percent of the 562 plots had severe rot, while 6% showed some level of resistance. Visual ratings correlated with quantitative assessments.  Significant correlations were found between the 2003 and 2004 ratings (r = 0.80) and counts (r = 0.75).  Three of the resistant accessions had previously been used in crosses and their progeny were planted in a large evaluation trial. Fungicides were withheld in 2005-2007 and the trial was evaluated each year for field fruit rot.  In 2007, disease pressure was so severe that of the 1644 progeny evaluated from 30 crosses (including four crosses with a resistant parent), 1085 progeny exhibited nearly 100% rot, while only 13 plots had a rating of ‘2’ (< 40% fruit rot).  Families with a resistant parent had a higher frequency of resistant progeny, indicating additive genetic effects and the potential for improving resistance through breeding. However, a few resistant progeny originated from susceptible parents suggesting non-additive variance for field fruit rot resistance also exists. Fruit cultured from susceptible and resistant cranberry plots had the same species of fruit rot fungi present, suggesting broad-based resistance. The current hypothesis is that the fruit rot fungi infect during bloom and then remain latent until the fruit ripening phase. Thus, in an effort to identify a mechanism of resistance, we evaluated flowering characteristics of resistant and susceptible accessions. No relationships were found between fruit rot resistance and bud development (starting at initial bud break), %bloom, or flowering period. Studies are now underway to evaluate the polyphenolic profiles of resistant accessions, starting early in fruit development. SCAR markers indicated that the resistant accessions in our germplasm collection are genetically diverse, offering multiple sources of resistance. The fruit rot-resistant plants have now been used in over 60 crosses with the ultimate objective of developing high-yielding cranberry varieties with increased levels of fruit rot resistance.