Phytophthora control is used to protect susceptible plants



   The genus Phytophthora belongs to the Family Oomycetes (water molds) which cause significant diseases in many plant species.  The Phytophthora genus includes about 150 named species (Scott et al, 2013) which can cause significant economic losses on crops worldwide (Erwin & Ribeiro 1996), as well as environmental damage in the south-west of Western Australia where approximately 40% of the 5710-described species are susceptible to P. cinnamomi (Shearer et al 2004). Phytophthora cinnamomi is one of the world’s most invasive species, having a wide host range. Whilst the recently described species P. pseudocryptogea that belongs to the P. cryptogea species complex (Safaiefarahani et al, 2015) is poorly understood with regards to its host range and distribution worldwide.

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Although controlling any soilborne plant pathogen is difficult, methods of control for Phytophthora diseases include biological, chemical and cultural methods (Ahmed et al, 1999). Chemical control is most commonly used due to its speediness, high efficiency and cost-effectiveness (Gisi, 2002). Although biological control tends to limited in its use, and is mainly applicable to agriculture and horticulture but not natural ecosystems. Cultural techniques can also be used, these include drainage, sanitation, and clean plant stock, all of which can reduce the impact and severity of Phytophthora diseases, but cannot result in total control (Thomidis, 2010).

Chemical control is used to protect susceptible plants through the application of fungicides. Fungicides can be either contact, translaminar or systemic (Kromann et al, 2008). Contact fungicides are not taken up into the plant tissue and protect only the plant where the fungicide

spray is deposited. Translaminar fungicides redistribute the fungicide from the upper, sprayed leaf surface to the lower, unsprayed surface (Rouabhi, 2010). Systemic fungicides are taken up and redistributed through the xylem vessels (Erwin, 1973). Few fungicides move to all parts of a plant. Some are locally systemic, and some move throughout the plant.

  The major fungicide groups used to control Phytophthora diseases include the phenylamide fungicides (PAFs), demethylation inhibitors (DMIs) in sterol biosynthesis and fungicides of the strobilurin-type (Gisi and Cohen, 1996).  Recently, several new fungicides have been developed with known activity against oomycete pathogens. The most effective is phosphite (phosphonate), the anionic form of phosphonic acid (HPO32–), which has been shown to control many plant diseases caused by Phytophthora (Hardy et al 2001). It is a systemic fungicide and effectively controls Phytophthora diseases in many horticultural and ornamental crops (Merinoa, 2015).  It can be applied as a soil drench, foliar spray or by trunk injection (Guest and Grant, 1991; Hardy et al, 2001). In the past, the Department of Conservation and Land Management (CALM) in Western Australia has trialed the aerial application of phosphite to protect small areas of native bushland from Phytophthora cinnamomi (Komorek and Shearer, 1997). Once phosphite enters the plant it is translocated throughout the plant and accumulates at actively growing sites (Whiley et al, 1995). But excessive phosphite concentrations have resulted in phytotoxicity in horticultural crops (Seymour et al. 1994) and native species (Tynan et al. 2001). 

  The classical phenylamide fungicide, metalaxyl (Subdue) was introduced in the 1970s as a systemic fungicide and has been used extensively against Phytophthora diseases (Farih et al, 1981). Metalaxyl has been shown to control Phytophthora gummosis reducing the size of stem lesions when applied as a soil drench and stem treatment (Farih et al. 1981). Metalaxyl is highly inhibitory to in vitro mycelial growth, which is involved in the initial infection and the subsequent lesion development of Phytophthora (Farih et al. 1981).

  But the increasing popularity of phenylamide fungicides by mid-1990s reduced the effectiveness of metalaxyl in potato crops as resistant genotypes of P. infestans were reported (Goodwin et al. 1996). Since then, metalaxyl-resistant isolates of P. infestans have been described in many regions around the world (Zavala et al, 2017).  The best control of Phytophthora on ornamentals occurs when metalaxyl is applied as a soil drench. Currently, there are relatively few other popular fungicides available for controlling Phytophthora diseases in plants. Those that are available include Propamocarb (Previcur N), etridiazole (Terrazole and Truban), fosetyl aluminum (Aliette), all of which have been tested on one or more ornamental crops (Chase and Mellich, 1992). All of these fungicides have been applied to the potting medium with the exception of Aliette, which can be applied either as a drench or a foliar spray (Chase, 2017). Although some differences in efficacy occur from plant to plant, these fungicides are effective (Chase, 2017).

  Aliette as a drench application is sometimes more effective than metalaxyl (Chase, 2002). However, when Aliette is applied as a foliar spray, the efficacy is often a little lower than a drench of either metalaxyl or Aliette (Chase, 2017). Terrazole and Truban drenches are also efficient for Phytophthora, but usually a little less effective than metalaxyl drenches (Butler, F. 1991).

  In most cases, the application of these fungicides to a disease, controls the majority of the Phytophthora species present. However, less sensitive and resistant individuals may exist in low frequencies and these may not be controlled as effectively or not controlled at all (Gisi et al, 2000). The selection process imposed by fungicides is initiated by the survival and subsequent increase in frequency of less sensitive and more resistant individuals (Gisi et al, 2000). They may become an important part of the population as long as the selection pressure persists but decrease in frequency once fungicide applications are ceased (Gisi et al, 2000). On the other hand, with immoderate use of efficient fungicides, the process of resistance might be a potential threat for a long lasting useful life of them (Davidse, 1981).

  Because of serious resistance to current fungicides, there is a need to develop new fungicides to ensure on-going protection of plants to disease.  With the advancement in the field of bioinorganic chemistry, the role of transition metal complexes as therapeutic compounds has become increasingly important (Atheer,  Date).

   Antifungal activities of putative fungicides are normally evaluated by their ability to prevent fungal growth in vitro. Chelates are more stable than non-chelated compounds of comparable composition, and the more extensive the chelation—that is, the larger the number of ring closures to a metal atom—the more stable the compound (REF). This phenomenon is called the chelate effect; it is generally attributed to an increase in the thermodynamic quantity called entropy that accompanies chelation (Martell, 1994). The stability of a chelate is also related to the number of atoms in the chelate ring. In general, chelates containing five- or six-membered rings are more stable than chelates with four-, seven-, or eight-membered rings (Martell, 1994).

  The aim of current study was to test a chelated compound calcium complex against P. cinnamomi and P. pseudocryptogea in planta, and compare its effectiveness to phosphite. Two experiments were conducted.  The first used one-day–old Lupinus angustifolius seedlings with P. cinnamomi, and the second used one-year old Banksia grandis and B. littoralis plants.

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