Laboratory Evaluation of Indigenous North American Fungi for Biological Control of Purple Loosestrife

Fifteen fungi were tested for pathogenicity to 6-week-old purple loosestrife plants (Lythrum salicaria) grown from seed in growth chambers. Virulence, based on percentage of plant surface displaying symptoms, was rated from 0 = no symptoms to 5 = dead plant. A fungus was considered virulent and a potential biological control agent when it caused a disease rating of 2.50 and above. On this basis, 6 of the 15 fungi were virulent, withSeptoria lythrinabeing the most virulent followed byAlternaria alternata, Botrytis cinerea, Colletotrichum truncatum, Phoma sorghina,andConiothyrium olivaceum.Inoculation of purple loosestrife seedlings was most effective when spores were applied in a carrier (referred to as DIGS) composed of 0.1% dextrose; 1.5% Intac (a blend of polymers and copolymers); 0.05% gelatin (Know, unflavored); and 0.15% Silwet L-77 (a silicone–polyether copolymer). The virulence ofB. cinereawas reduced when stored cultures (0–16 months at 5°C) were used, but was regained when the fungus was reisolated from leaf spots. There was a difference in virulence amongA. alternataandB. cinereaisolates from different locations. The mean disease ratings ofA. alternatawhen inoculated on 4-or 6-week-old plants were the same; however, disease ratings withB. cinereawere greater on 6-than on 4-week-old plants. Thus, based on these laboratory studies,A. alternata, B. cinerea,andP. sorghinaapplied in DIGS as a carrier may have potential for biological control of purple loosestrife.