Selection of fungal biological control agents of Sclerotium cepivorum for control of white rot by sclerotial degradation in a UK soil

Abstract

A range of fungal isolates was tested in a three-stage screening system for their ability to degrade sclerotia of Sclerotium cepivorum on agar and in soil, and to reduce white rot disease on onion seedlings. Biological control agents (BCAs) were identified that could degrade up to 60% of sclerotia in soil and significantly reduce white rot disease on onion seedlings. The efficacy of the BCAs was enhanced when applied as wheat bran cultures compared with spore suspensions, and two of the best BCAs from the screening procedures were both identified as Trichoderma viride (L4, S17A). When L4 and S17A were fluid-drilled in guar gum with bulb onion seed in the field white rot symptoms were significantly reduced, but stem base applications applied mid-season had little effect. The strategy of selecting and using BCAs that degrade sclerotia of S. cepivorum and integration with other control methods is discussed.

White rot (Sclerotium cepivorum Berk) - an aggressive pest of onion and garlic in Ethiopia: An overview

“…Similar work in the United Kingdom has identified two strains of Trichoderma viride that provided significant protection against white rot, at levels comparable to tebuconazole (Clarkson et al, 2002). In the same country, other workers found potential in Coniothyrium minitans for control of white rot equal to that provided by calomel, without the phyto-and environmental toxicity associated with that chemical's use (Ahmed and Tribe, 1977).…”

“…In addition to limited effectiveness, the phenomenon of enhanced degradation of dicarboximide fungicides by resident soil micro-organisms has been detected in fields where such fungicides have been used regularly, with some research identifying a 90% loss of dicarboximide fungicides in soils pre-treated with them within 7 days (Garcia-Cazorla and Xirau-Vayreda, 2005). With the number of sites where onions and garlic can be readily grown so small, and the proportion of those sites infested with white rot increasing, the gradual selection of soil microbes capable of enhanced degradation of dicarboximide fungicides in those soils (Athiel et al, 1995) makes the use of such fungicides ultimately a finite solution (Clarkson et al, 2002). An alternative fungicide, tebuconazole, has offered hope in the control of white rot in infested fields, showing better control than the best dicarboximide fungicide procymidone in some comparisons (Duff et al, 2001).…”

“…However, tebuconazole is best suited for foliar spraying, showing phytotoxicity when applied as a seed dressing (Fullerton et al, 1995), though granular applications may avoid this (Fullerton et al, 1995). Tebuconazole and triadimenol, both triazole fungicides, may well hold the most promise for ongoing control of white rot (Tyson et al, 1999;Clarkson et al, 2002), with no evidence yet arising to suggest that enhanced degradation of these fungicides occurs in the field (Pung et al, 2007). However, there is no reason to suppose that soil microbes in treated areas will not eventually develop the ability to degrade these fungicides at an enhanced rate, particularly if farmers become more reliant on their use, thereby selecting more heavily for soil microbes that can degrade them.…”

“…Then, if previously-infested fields are planted with Allium species, the use of fungicidal seed coats, dips and foliar sprays reduces the incidence of disease in those fields, while biological control agents work antagonistically against the sclerotia in the soil. The use of resistant cultivars could also provide another weapon in this arsenal, but no such cultivars have been developed to a commercial standard (Clarkson et al, 2002). The use of an Allium cultivar specifically and directly transformed to express a gene for white rot resistance might allow these other control methods to work more effectively, or even rule out the need for some of the more economically and environmentally expensive control methods, such as soil.…”

<i>In vitro </i>technique for selecting onion for white rot disease resistance

“…Various chemical, biological, and physical measures have been examined for the control of the disease, including fungicides (Zewide et al, 2007a), soil amendments (Brewster, 2008), soil solarisation (Melero-Vara et al, 2000), composts (Smolinska, 2000;Coventry et al, 2005) and biological control agents (Clarkson et al, 2002;Coventry et al, 2006;Zhang et al, 2013;Elsherbiny et al, 2015;Mahdizadehnaraghi et al, 2015). In addition, a number of studies have attempted to mimic the natural phenomenon of sclerotia germination with chemicals (Davis et al, 2007).…”