90 Percival and Noviss: Potassium Phosphite and Myclobutanil for Pear Scab (Venturia pirina) Suppresion DISCUSSION Results of this study demonstrated that Pi at 10 ml and 20 ml per liter of water applied at four growth stages (bud break; green cluster; 90% petal fall; early fruitlet) under field condi- tions significantly reduced the incidence and severity of pear scab. Previous studies have reported that application of Pi to be extremely effective in the management of pathogens, particularly those that belong to the Oomycetes group such as Phytophthora citricola, P. capsici, P. infestans, Peronosclerospora sorghi, Peronospora parasitica in several economically impor- tant food crops to include potato, maize, pepper, and crucifers (Ouimette and Coffey 1989; Jackson et al. 2000; Wilkinson et al. 2001; Miller et al. 2006). In addition, phosphite applica- tion reduced pathogen severity of several powdery mildews to include Sphaerotheca fuliginea (cucumber powdery mildew), grape powdery mildew, nectarine, and mango powdery mildew (Bécot et al. 2000), as well as suppress the bacterial pathogen Pseudomonas syringae pv. syringae (pear blast) (Moragrega et al. 1998). Within Australia, phosphites are now extensively used for the control of Phytophthora cinnamomi in avocados, Downy Mildew (Plasmopara viticola) in grapevines, and Phytophthora cinnamomi and P. parasitica (heart rot) in pineapples. Howev- er, to the best knowledge of the authors of the present study, this is the first report of the efficacy of Pi against V. pirina. In the two field experiments conducted on Pyrus communis ‘Williams’ Bon Chrétien’, a highly sensitive pear scab variety, applications of Pi at 20 ml per liter of water significantly and consistently reduced pear scab incidence and severity on leaves and fruit to a greater degree than Pi applied at 10 ml per liter of water. Such a response was recorded even when greater conditions of rainfall were more conducive for scab development in 2007. Manufacturers recommend a 10 ml per liter dose of Pi for pathogen suppression and/or stimulation of plant vitality. Results of this study indicate an increase in Pi concentration to 20 ml per liter of water can achieve a greater degree of scab reduction without phytotoxicity symp- toms developing on leaves or fruit. In addition, Pi at this higher concentration resulted in a greater fruit yield and crown vol- ume compared to Pi applied at 10 ml per liter of water and stand alone applications of a reduced dose of myclobutanil. The combined effects of Pi at 10 ml and 20 ml per liter of water with a reduced dose of myclobutanil were more than their additive properties according to Limpel’’ s formula. Pi and a reduced rate of myclobutanil acted synergistically in both the July and September assessments to reduce V. pirina incidence and severity. Such synergistic effects were particu- larly pronounced with respect to crown volume and fruit yield recorded in September 2006 and 2007. Consequently, com- bining Pi’s with higher concentra tions than the reduced dose of myclobutanil used in this study may result in even greater synergistic activity. Such a hypothesis is given credence by the fact that myclobutanil applied at full strength at bud break, green cluster, petal fall, and early fruitlet formation proved the most effective treatment for pear scab control. Under these cir- cumstances leaf and fruit scab severity was reduced by 75%–85%. Such a result is comparable with other workers following similar spray regimes [i.e., four times between flower bud formation to initial early fruit set (Butt et al. 1990; Swait and Butt 1990; Bevan and Knight 2001). Within the UK, total scab control is gener- ally achieved via a minimum of 10–12 fungicide sprays through- out the growing season (Butt et al. 1990; Swait and Butt 1990)]. Irrespective of year, the crown volume, number of fruit per tree, and fruit yield were in all cases higher in Pi and myclobuta- nil treated trees irrespective of concentration applied compared to water-treated controls. However, little effects on mean fruit weight per tree were recorded. Detrimental effects of scab in- fection include premature fruit drop and reduction in fruit size (Agrios 2004; Villalta et al. 2004). Results of this investigation indicate reductions in fruit yield were caused by scab-induced fruit drop rather than specific reductions in individual fruit size. Such a response may be related to source-sink relationships. Scab induced premature fruit drop would mean the smaller re- maining number of fruit having a greater sink strength that in turn would account for their comparable mean weight com- pared to Pi and myclobutanil-treated trees (Blanke 2007). From a commercial aspect, producers, suppliers and ven- dors of pears adopt a zero tolerance policy toward pear scab on fruit (Butt et al. 1990). Consequently, to reduce scab levels to commercially accepted standards, several follow-up fungicide sprays would need to be applied. However, where fruit pro- duce is sold under a naturally produced label within the UK, scab severity levels tend to be less stringent (Bevan and Knight 2001). Likewise, ornamental pears are grown and planted for aesthetic reasons within town and city landscapes where lower scab levels are acceptable (Percival and Haynes 2008). In these instances, Pi application can possibly be used alone or in ro- tation with synthetic fungicides as part of an IPM strategy. In conclusion, results of this study indicate that applica- tion of Pi alone is useful in reducing the incidence and severity of pear scab. Combination of Pi with a reduced dose of myclob- utanil, however, significantly improved the efficacy compared to stand alone applications of each product to further reduce the incidence and severity of pear scab on leaves and fruit. Such a response indicates that Pi and myclobutanil combinations of- fer potential to reduce the risk of fungicide resistant strains of scab developing by decreasing fungicide selection pressure. This is based on the fact that higher degrees of scab control were achieved with lower myclobutanil inputs when combined with Pi. These findings would be useful to arborists as they indicate synthetic fungicide usage can be reduced yet achieve control comparable or significantly higher than stand alone appli- cations of fungicides at full dose. This in turn promotes a greater awareness of environmental and health issues to the public and government legislative bodies. Likewise phosphites cost in general 40%–80% less than conventional fungicides. Further research is ongoing evaluating Pi combinations with higher doses of myclobutanil and other forms of synthetic fungicides. Acknowledgments. The authors are grateful for funding, in part, from the TREE FUND (Hyland Johns Grant). ©2010 International Society of Arboriculture
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