Characterization of blue mold caused by Penicillium spp. in cold stored table grapes

High production of table grapes ( Vitis vinifera ) in Chile, particularly the cultivar ‘Red Globe’, often necessitates storage beyond 60 days at -0.5°C before the grapes could be sold. This has promoted the development of blue mold ( Penicillium expansum ), causing watery soft rot and internal breakdown on cold stored grapes. A relative low decay incidence (<2%) has commonly been determined. However, this has considerably limited the commercialization of Chilean table grapes at the international markets. The objectives of this research were: i. To characterize and identified the species of Pencillium associated to table grapes in Chile, ii. To study the population dynamic of Penicillium spp . on table grapes under commercial field conditions, iii. To study the effect of high content of total soluble solids (TSS) on disease development, and iv. To study the ability of Penicillium spp . to grow and to infect table grapes under cold conditions. Thirty two isolates of Penicillium were collected from Copiapo and Nancagua (27o18’ and 34°65’ South lat., respectively) in 2004-2005 growing season. Based on morphological features and growth characteristics P. expansum and P . chrysogenum were identified. Populations of Penicillium were low on apparently healthy ‘Thompson Seedless’ berries at the beginning of berry development (5% TSS). However, populations considerably increased at harvest (17% TSS) time. Water activity (a w ) below 0.96 reduced mycelial growth of P. expansum and P. chrysogenum . Lesion size was reduced on berries of 23% TSS berries. Four of five P . expansum isolates grew in vitro 0°C having measurable growth after eight days. Similarly, these isolates infected ‘Red Globe’ and ‘Crimson Seedless’ table grapes at 0oC, developing first symptoms after 30 days of incubation. These results demonstrated that P. expansum can infect and develop symptoms after a long storage at 0oC. It is possible that table grapes become surface contaminate with conidia of P. expansum before harvest. This is the first report of P . chrysogenum on table grapes in Chile; however, its pathological role remains to be determined. Penicillium expansum , es el agente causal del moho azul y de pudriciones blandas y acuosas que deterioran completamente bayas de uva de mesa ( Vitis vinifera ) conservadas por largo tiempo a 0°C. En los ultimos anos ha adquirido importancia debido a la necesidad de conservar uvas Red Globe a 0°C por mas 60 dias, en espera de su comercializacion. Esto ha favorecido la presencia de este patogeno. Este trabajo tuvo el proposito de caracterizar e identificar las especies de Penicillium en uva de mesa en Chile, estudiar la dinamica poblacional de Penicillium en condiciones de campo. Al mismo tiempo se determino el efecto de altos contenidos de solidos solubles totales (SST) en el desarrollo de P . expansum y se estudio el efecto de la refrigeracion a 0oC sobre el desarrollo de P . expansum in vitro e in vivo . De los once aislamientos obtenidos entre Copiapo (27o18 latitud sur y 70o25 de longitud oeste) y Nancagua (34°65 latitud sur con 71°20 longitud oeste) cinco fueron identificados como P . expansum y seis correspondieron a P . chrysogenum . Se ratifico la presencia de Penicillium spp . sobre bayas siendo maximo en el momento de la cosecha (17% SST) con 215,7 ufc·g -1 en bayas cv. ‘Thompson Seedless’. Se comprobo que la actividad del agua (a w ) de 0,96 disminuye el crecimiento miceliar de P. expansum y P. chrysogenum , esto explicaria, en parte, la menor agresividad con SST mayores a 23%. En condiciones de almacenaje en frio, P. expansum crecio a 0°C luego de ocho dias en agar papa dextrosa y a los 30 dias sobre las bayas de los cvs. ‘Red Globe’ y ‘Crimson Seedless’. En conclusion, este trabajo demostro que P. expansum tiene la capacidad de crecer y podrir bayas de uva de mesa en condiciones de almacenaje a 0 °C. Es posible que los racimos sean superficialmente contaminados durante la cosecha aportando el inoculo necesario para infecciones en postcosecha. Esta es la primera mencion de P. chrysogenum en uva de mesa en Chile.