EFFECTS OF COLD SHOCK ON THE SUSCEPTIBILITY OF WHITE LUPINS TO INFECTION BYFUSARIUM AVENACEUM AND PLEIOCHAETA SETOSA
JV ETHERIDGE and GL BATEMAN
IACR-Rothamsted, Harpenden, Hertfordshire. AL5 2JQ, UK
Background and objectives
The white lupin (Lupinus albus L.) has considerable potential as a new high-quality protein crop in the UK. Genotypes being developed for autumn-sowing are winter-hardy but substantial overwinter losses can occur . Severe frost is the principal cause but evidence suggests that plant pathogenic fungi may also be involved. Pathogenic fungi associated with plants dying overwinter include Fusarium avenaceum and Pieiochaeta setosa . This study examined, under controlled environment conditions, how interactions between cold injury and infection by fungal pathogens may contribute to overwinter losses of the white lupin.
Materials and methods
Plants were grown in wooden troughs (1.2 x 0.27 x 0.4m) insulated with polystyrene foam sheets and filled with a John lnnes No. 1 compost/vermiculite mix (213:113 volume ratio). The troughs were placed in a Saxcil controlled environment cabinet maintained on a 12o/6 oC 12-h day/12-h night cycle. The compost was frozen when required by passing glycol, from a refrigeration plant, through two plastic--coated copper pipes that extended the length of each trough. Before freezing, troughs were covered with a layer of vermiculite to decrease heat exchange; this was removed after freezing. Vermiculite was also applied to control troughs which were not frozen. For the freezing treatment, the temperature of the compost was cooled to -1 oC and maintained at this temperature for 4 d.
Troughs were sown with pregerminated white lupin seed (cv. Lucyane). The radicle of each seedling was dipped into a Rhizobium suspension and planted 5 cm deep, in three rows, with a total of 72 seeds/trough. In Experiment 1, the length of the trough was divided into four equal sections each containing three rows of six seeds. Each section was sown at a different time to give plants of four different ages at freezing. Each row within a section received a different inoculum treatment and each row was further subdivided so that three of the plants received pre-inoculation wounding and three did not. Plants were inoculated at planting by applying a PDA plug of either Fusarium avenaceum or Pieiochaeta setosa to the hypocotyl base of each seedling. Uncolonized PDA plugs were used as a control treatment. In Experiment 2, the length of the trough was subdivided as before into four sections. Seeds were sown to give plants of two ages at freezing. The roots of seedlings were inoculated either at the time of sowing or after soil freezing with one of two isolates of F.avenaceum or a PDA control. Plants were assessed for disease 8 weeks after soil freezing. Disease was scored on a 0-5 scale (0, no disease; 5, most severe disease); the number of leaves per plant and the shoot length (cm) were also recorded.
Results and conclusions
Soil freezing exacerbated injury of the youngest (10d old) plants caused by hypocotyl infections by Pieiochaeta setosa, 38% of which died. Less than 5% of older plants inoculated with P.setosa died after soil freezing. Wounding the hypocotyl increased the severity of infection caused by P.setosa.
Soil freezing had no significant effect on plant death when either the hypocotyl or roots were inoculated with Fusarium avenaceum. Inoculation time had a significant effect: inoculating F.avenaceum on roots before freezing caused more severe disease than inoculating after freezing.
Frost damage and infection by certain plant pathogens may interact, therefore, to cause lupin plant death, particularly in young plants. This adds to evidence  that, under UK conditions, early sowing is necessary for plants to become sufficiently well developed to survive frost.
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