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their host root, attach, retract their flagellae and excrete a cell wall forming a cyst. The cyst then germinates on the outside of the root, penetrates and begins to cause root rot. We demonstrated that very low levels of cellulytic enzymes will completely prevent the encystment of spores on avocado roots. There are many mechanisms for control of root disease by mulch. Mulch effects such as increased porosity (aeration) increased infiltration rate, reduced runoff, increased biology and antagonistic potential from saprophytic microbes as well as the increase in mycorrhizae and soil enzymes all play a role in limiting the development of Phytophthora root rots. When you look at the biology of mulch systems it is very hard to tie the successful system a single mechanism of action. Mulch has a very complicated ecology that for a time can control pathogens and help create a suppressive soil. It should be noted though that mulch decays, it wears out, it needs to be replenished-- either by the trees growing in and under it or by continued mulch application. Also, mulch systems are not a panacea for root rot control. If not carefully managed, especially as they relate to soil moisture content, they can exacerbate rather than control disease. Finally, mulch systems are ideally used as part of an integrated system in concert with fungicides, host resistance and cultural practices for a complete root rot control package. 

Literature Cited Broadbent, P. and K.F. Baker. 1974. Behavior of Phytophthora cinnamomi in soils suppressive and conducive to root rot. Aust. J. Agric. Res. 25:121-137. Bedini, S., E. Pellegrino, L. Avio., S. Pellegrini, P. Brazzoffi, E. Argese and M. Giovannetti. 2009. Changes in soil aggregation and glomalin-related soil protein content as affected by the arbuscular mycorrhizal fungal species Glomus mosseae and Glomus intraradices. Cook, R.J. 1990. Twenty Five years of progress towards biological control. In Biological Control of Soil-Borne Plant Pathogens. D. Hornby Ed. CAB International Wallingford UK. Downer, A.J., J.A. Menge, and, E. Pond, 2001. Effects of cellulytic enzymes on Phytophthora cinnamomi Rands. Phytopathology: 91: 839-846 Downer, A.J., J.A. Menge, and E Pond. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi Rands. Phytopathology: 91 847-855 Linderman. R.G. 1988. Mycorrhizal interaction with the rhizopshere microflora: The mycorrhizosphere Effect. Phytopathology 78:366-371. Marx, D.H., Davey, C.B. 1969. The influence of ectotrophic mycorrhizal fungi on the resistance of pine roots to pathogenic infections. IV. Resistance of naturally occuriring mycorrhizae to infections by Phytophthora cinnamomi. Phytopathology 59: 559-565.

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December CAPCA Adviser  

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