Botrytis cinerea inoculation on 3- and 6-month-old Norway spruce seedlings was tested in combinations of temperature (15 and 25°C) and relative humidity (50 and 80%) in a growth chamber. Occurrence of needle damage was examined 11 days after the inoculation. Damage occurred more on inoculated than on control seedlings. Needle damage tended to occur proportionally more on the older seedlings and concentrating more on the tops of the shoot in the younger seedlings. The higher temperature suggested slightly more damage occurrence than the lower temperature did. The relative humidity did not appear to correlate with the damage outbreak. Surface wetness tended to increase the damage occurrence but so did also the vapour pressure deficit. At the lower relative humidity, the seedlings had to be irrigated more frequently, which increased the surface wetness. Thus, the results suggest that any irrigation method or other condition control that can provide low surface wetness on seedlings decreases needle damage by B. cinerea in forest nurseries. 1. Introduction The gray mold fungus (Botrytis cinerea Pers.:？Fr.) is a facultative parasite. It lives as a saprophyte on dead plant material and is also a plant pathogen. The temperature optimum for gray mold spore germination and infection is 9–21°C, but temperatures between 2–25°C are also suitable for the fungus [1–4]. Relative humidity greater than 98% and free water are optimal for spore germination and mycelia growth of the B. cinerea fungus, for example, [5]. In Finland, Norway spruce (Picea abies (L.) Karst.) is the predominant species in forest nursery seedling production. In recent decades, container seedling production has increased and there has also been a subsequent increase in damage caused by gray mold [6, 7]. This increase is partly due to higher seedling densities and to microclimatic conditions, which differ from those in bare root seedling production. The damage may occur only few days after inoculation on vital needles [8]. Visual symptoms are spots of discoloration usually in the middle of the needles or completely dead needles [8]. Numerous studies have been carried out on gray mold on agricultural crops, while fewer studies have dealt with gray mold on conifer seedlings, for example, Picea mariana (Mill.) B.S.P, [9, 10], Sequoiadendron giganteum (Lindl.) Dene [11], Pseudotsuga menziesii (Mirb.) Franco [5] 1988, and Larix occidentalis (Nuttall) [12]. In the nursery, gray mold development is known to be dependent on the growth phase of the seedlings [8] but also markedly on the ambient microclimate
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