Effects of Temperature and Wetness on Components of the Infection Cycle of Valdensia heterodoxa in Lowbush Blueberry.

Valdensia leaf spot, caused by Valdensia heterodoxa, is a serious disease of lowbush blueberry. The disease may develop rapidly, resulting in extensive defoliation of fields. The purpose of this study was to examine the effects of temperature and wetness duration on various components of the infection cycle to gain a better understanding of epidemic development that might lead to improved management practices. Lesions on leaves appeared 6 h after inoculation at 20°C and were larger on young 3-week-old leaves compared with 8-week-old leaves. Incidence of infection on 3-week-old leaves was lowest at 5°C, highest at 15 and 20°C, and failed to occur at 30°C. Defoliation began 48 h after inoculation at 20 and 25°C but was slower at higher and lower temperatures. Conidia production and release from colonized leaves began 48 h after inoculation at 15 and 19°C. Total conidia production was lowest at 7°C, highest at 15°C, and progressively declined at 19 and 23°C. Production of conidia lasted 2 to 3 days. Sclerotia formed mainly along the midveins and were similar in size at 5 to 15°C, largest at 20°C, and smallest at 25°C. Conidia formed directly on sclerotia that were overwintered outdoors and then incubated on moist filter paper. Conidia production began after 48 h at 10, 15, and 20°C. Total production was lowest at 5°C, highest at 20°C, failed to occur at 25°C, and ceased after 10 days at all temperatures. These data show that at optimal temperatures, relatively short wet periods are required for conidia production on overwintered sclerotia, infection of leaves, and subsequent conidia production on diseased leaves that may account for the sudden and rapid spread of disease in fields. The data will be useful for helping growers identify weather conditions favorable for disease development.

In vivo study of developmental programmed cell death using the lace plant (Aponogeton madagascariensis; Aponogetonaceae) leaf model system.

Programmed cell death (PCD) is required for many morphological changes, but in plants it has been studied in much less detail than in animals. The unique structure and physiology of the lace plant (Aponogeton madagascariensis) is well suited for the in vivo study of developmental PCD. Live streaming video and quantitative analysis, coupled with transmission electron microscopy, were used to better understand the PCD sequence, with an emphasis on the chloroplasts. Dividing, dumbbell-shaped chloroplasts persisted until the late stages of PCD. However, the average size and number of chloroplasts, and the starch granules associated with them, declined steadily in a manner reminiscent of leaf senescence, but distinct from PCD described in the Zinnia tracheary element system. Remaining chloroplasts often formed a ring around the nucleus. Transvacuolar strands, which appeared to be associated with chloroplast transport, first increased and then decreased. Mitochondrial streaming ceased abruptly during the late stages of PCD, apparently due to tonoplast rupture. This rupture occurred shortly before the rapid degradation of the nucleus and plasma membrane collapse, in a manner also reminiscent of the Zinnia model. The presence of numerous objects in the vacuoles suggests increased macro-autophagy before cell death. These objects were rarely observed in cells not undergoing PCD.