BALTIC FORESTRY ARTIFICIAL INFECTION AND DEVELOPMENT OF SNOW MOLD FUNGUS /.../ R.-L. PETÄISTÖ ET AL. Artificial Infection and Development of Snow Mold Fungus (Phacidium infestans) in Container-grown Norway Spruce Seedlings RAIJA-LIISA PETÄISTÖ*1, ARJA LILJA2 AND JARKKO HANTULA2 1 Finnish Forest Research Institute, Suonenjoki Research Unit, 77600 Suonenjoki, Finland, 2 Finnish Forest Research Institute, Vantaa Research Unit, PO Box 18, 01301 Vantaa, Finland. *Corresponding author. E-mail: [email protected] Petäistö, R.-L., Lilja, A. and Hantula, J. 2013. Artificial Infection and Development of Snow Mold Fungus (Phacidium infestans) in Container-grown Norway Spruce Seedlings. Baltic Forestry 19(1): 3138. Abstract Phacidium infestans causes common snow mold in Scots pine (Pinus sylvestris L.), its main host in Finland. Recently, a mycelial web similar to that occurring on pine has been observed on Norway spruce (Picea abies L.) seedlings in some forest nurseries of Finland. In this study, we showed that Ph. infestans can cause snow mold in container seedlings of Norway spruce exposed to treatments that simulated natural infection by ascospores borne on Scots pine saplings. In the following spring after infection, inoculated seedlings stored in the freezer (-3 °C) were generally more diseased than those stored outdoors during the 2006/2007 winter, suggesting that Ph. infestans does not require snow cover to develop on spruce seedlings. Diseased needles were grey-green in early spring. After death, diseased needles soon became yellow- brown or grey-brown and seedlings often died. In contrast to the disease in Scots pine of the same age, infected Norway spruce needles were dropped mainly during the summer of 2007. Although the final examination took place about two years after artificial infection, we did not observe any mature fruiting bodies of Ph. infestans on spruce or pine seedlings included in the experiment as a reference. Key words: Phacidium; nursery practice; snow blight; ascospore infection; overwintering; Picea Introduction Technology and growing practice in commercial seedling production have changed considerably over The snow mold or snow blight fungus, Phacid- the last 2030 years in Finland (Tervo 1999). Today, ium infestans P. Karst. (Phacidiaceae, Ascomycota), is over 99% of seedlings are grown in containers, ger- a common and serious pathogen of Pinus in parts of minated in greenhouses and later transferred outdoors Eurasia, North America and Japan that experience pro- (Lilja et al. 2010). Two thirds (about 100 million) of longed snow cover (Petrak 1955, Bazzigher 1978). The seedlings produced are Norway spruce (Picea abies fungus also infects Abies, Picea and Juniperus grow- Karst) (Finnish Statistical Yearbook 2011). Approxi- ing close to diseased pines (Björkman 1948, Roll- mately one third of seedlings are stored in freezers at Hansen 1987, 1989). In Baltic forests, Ph. infestans is about -3 °C over winter and the rest outdoors under known to occasionally cause damage during winters natural or artificial snow cover (Lilja et al. 2010). with thin and short-term snow cover (Hanso 2000, Recent years have brought long, warm and humid McLaughlin and Ðica 1993). autumns that encourage infection by pathogens such Snow mold ascospores spread during wet weath- as Ph. infestans (Björkman 1949, Jylhä et al. 2009). In er with high relative humidity (RH). Dispersal begins some nurseries, groups of diseased Norway spruce in September and continues until the apothecia are seedlings have been found after snowmelt with symp- covered by snow or the spore production capacity is toms typical of Scots pine seedlings damaged by Ph. exhausted (Kurkela 1996). Growth of Ph. infestans infestans. In response to this, we sought to determine occurs as low as -5 °C and beyond +25 °C (Björkman whether Ph. infestans pose a risk to Norway spruce 1948, Butin and Söderholm 1984), with the optimum seedlings stored outdoors over winter or in the con- being +15 °C at 98100 % RH. Thus, conditions at stant, snow-free environment of a freezer. This study ground level under thick snow cover and during snow- also aimed to describe disease symptoms on Norway melt enable infections to develop and appear in the spruce seedlings, a new putative host of this patho- spring (Keränen 1920, Vuorinen and Kurkela 1993). gen in Finnish nurseries. 2013, Vol. 19, No. 1 (36) ISSN 2029-9230 31 BALTIC FORESTRY ARTIFICIAL INFECTION AND DEVELOPMENT OF SNOW MOLD FUNGUS /.../ R.-L. PETÄISTÖ ET AL. Material and methods unheated greenhouse about 50 m from a pine forest edge. One tray with 81 seedlings was a replicate and Seedling material each treatment was replicated eight times for Norway Experiments were performed at the Suonenjoki spruce and treatment 1 and 2 twice for Scots pine. At nursery of the Finnish Forest Research Institute. Qual- the time of inoculation, needle ascocarps were closed ified seed from seed orchards of Norway spruce (SV177 but had begun to open on 22 August and about 20% EY/FIN TO3-00-0446 useable for 1080-1280 d.d.) and were open a month later. Scots pine (SV337EY/FIN T03-03-0215) were sown on 25 April 2006 in PL-81F containers (81 seedling cells Winter storage treatments per tray, in 9 rows, each cell 85 cm3; Lannen, Iso-Vim- Norway spruce trays (81 seedlings/tray) repre- ma, Finland) filled with low-humified Sphagnum peat. senting each inoculation treatment with four replicates Seedlings of both conifer species were grown in a (12 trays together) and treatment 1 and 2 with one tray greenhouse prior to the experiment and moved out- of Scots pine, each representing one inoculation treat- doors 26 July 2006. Control seedlings remained in an ment were stored over winter outdoors and in a freez- unheated greenhouse to avoid natural infection. We er. Outdoor trays were randomly placed in three rows also measured the height of nine spruce in each tray spaced 5070 cm apart and embedded in a peat layer on 14 May and 29 September 2007 to monitor seed- to protect seedling roots against frost damage. In the ling growth during the summer after artificial infections freezer, each tray was packed into a separate cardboard performed the previous autumn (Figure 1). box and randomly placed on shelves. A temperature/ RH sensor (Hobo, MicroDAQ.com Ltd., PO Box 439, 200 Contoocook, NH 03229, USA) was placed between Outdoor storage seedlings in one tray in the freezer and in one tray Freezer storage outdoors. The freezer maintained a constant tempera- 150 ture of about -3 °C (Figure 2 a). The RH sensor in the cardboard box reported 100% in the first two days but subsequently malfunctioned. RH was most probably 100 100% during the entire winter as cardboard boxes were Growth, mm Growth, closed all the time and the temperature remained con- stant. Outdoor temperature, RH (average daily means) 50 and snow depth (measured on weekdays) during win- ter storage are shown in Fig. 2 b, c. The RH sensor typically showed about 100% RH. 0 Treatment 1 Treatment 2 Control Seedling condition after winter and second grow- Figure 1. Growth increment of Norway-spruce seedlings ing season during summer 2007 following infection treatment in 2006 Thawing of freezer-stored seedlings took place in and winter storage; measurements from nine seedlings in each April, with material increasing in temperature from -3 of four trays per treatment × storage combination to +4 °C. During the last week of thawing (27 April 2 May), seedlings were placed outdoors in a shaded Artificial infection with Ph. infestans ascospores location with ventilation holes of the boxes open. On In 2006, seedlings were exposed to inoculation treat- 4 May, inoculated seedlings were removed from the ments, one of them simulating infection by ascospores. boxes and evaluated, and likewise control seedlings Branches with diseased needles providing the inocu- on 89 May. Trays were moved to the nursery field lum were collected from Scots pine saplings in a near- adjacent to the seedlings that had spent the winter by forest that were naturally infected by Ph. infestans outdoors. Seedlings stored outdoors over winter were a year earlier. Treatments were: 1) artificial ascospore evaluated after snow had melted at the beginning of infection by placing Norway spruce seedlings under the April 2007. 23 layers of diseased Scots pine branches on an iron The condition of each Norway spruce seedling mesh covering the containers about 1.2 m above soil was evaluated individually. Each seedling shoot was level on August 16; 2) inoculation 1 + inoculation with divided into approximately equal quarters: 1 = top diseased needles at the time of transfer to winter stor- quarter, 2 = second quarter, 3 = third quarter and 4 = age (about 2 dl of pine needles bearing ascocarps were lowermost quarter. The condition of each quarter was scattered over each seedling tray on October 20); then classified: 1 = healthy; 2 = £10% diseased nee- 3) control seedlings were sheltered from infection in an dles; 3 = <50% diseased needles; 4 = >50% diseased 2013, Vol. 19, No. 1 (36) ISSN 2029-9230 32 BALTIC FORESTRY ARTIFICIAL INFECTION AND DEVELOPMENT OF SNOW MOLD FUNGUS /.../ R.-L. PETÄISTÖ ET AL. 10 needles; 5 = dead. Scots pine seedlings were observed more generally, i.e., evaluating each tray rather than 8 A individually. Spruce and pine needle samples were 6 collected from each tray for fungal isolations. Twelve C needles from four seedlings per inoculation/storage o 4 treatment were rinsed in 70% alcohol for 6 seconds and 2 three times with sterile water. Needle fragments of 2 3 mm were cut from each surface-sterilized needle and 0 placed on PDA-media (Potato dextrose agar, 39 g/l, Temperature, Temperature, Difco, MI, USA) in Petri dishes and incubated at 10 -2 15 °C in the dark. -4 A second inventory of spruce seedlings was com- pleted at the end of the 2007 growing season; 12 months -6 after seedlings were inoculated.