Abstract

Application of 2,4-dichlorophenoxyacetic acid (2,4-D) has been shown to be an effective control method for common scab of potato, but earlier research suggested that it can be ineffective in controlling disease if applied after tubers have begun to be susceptible to the disease. Treatments had previously been targeted to the start of the susceptible period, following tuber initiation. This study examines the effect of a range of application dates on disease control, in both Russet Burbank and Desiree cultivars, and presents the results of two field trials and two pot trials. The data obtained show that applications of 2,4-D as early as 5 days after plant emergence gives greater disease control than later treatments targeted to tuber initiation. In addition, these early treatments provide sufficient material to the tuber to induce resistance that lasts throughout the period in which the tuber is susceptible and result in levels of 2,4-D at harvest both well below the Australia maximum residue level, and lower than that of treatments applied later in the growing season.

2.2 Introduction

Common scab is a globally important soil borne disease of potato characterised by scab-like lesions on the tuber surface (Loria et al. 2006). It is caused by a number of pathogenic Streptomyces spp., most notably S. scabies. Pathogenic Streptomyces spp. produce a number of phytotoxins, called thaxtomins (King et al. 1989) which are essential for disease development. Common scab symptoms can be reproduced through the direct application of thaxtomins (Lawrence et al. 1990; Leiner et al.

1996). The capacity of Streptomyces spp. to produce thaxtomin A is correlated with their pathogenicity (Loria et al. 1995; Goyer et al. 1998; King et al. 2001; Kers et al.

2005). Thaxtomin production is therefore an important pathogenicity factor, and the reduction in its phytotoxic effect may be one method of controlling common scab. There is no single method for effectively controlling common scab. Disease control is currently attempted through a combination of strategies, including the

management of planting dates (Waterer 2002a; Wilson 2004), the planting of moderately resistant cultivars (Groza et al. 2005; Pasco et al. 2005; Wilson et al.

2010; Hiltunen et al. 2011) and strategic use of irrigation during early tuber development (Lapwood et al. 1973), the latter used extensively in the United Kingdom. Irrigation has not been as effective in Australia for management of

common scab (Wilson 2004), and increasingly scarce and expensive water resources worldwide require other management strategies to be sought. Despite various

adoptions of these practices, common scab occurs frequently and results in substantial losses around the world.

Treatment of potato seed tubers with fungicides or other biocides can be useful in some circumstances (Wilson et al. 1999), but there are no currently available effective soil-applied chemical treatments. Pentachloronitrobenzene (PCNB) was a soil-applied fungicide that did provide reasonable disease suppression and was widely used (Davis et al. 1974; Hooker 1981), however, the material is a known carcinogen and its use has been withdrawn from most potato production regions around the world. After the banning of PCNB in the United Kingdom, a series of trials were run between 1973 and 1978 to test a range of alternate chemicals for their ability to control common scab of potato in order to find a PCNB substitute. The trials initially focussed on materials incorporated into the soil with a number of

chemicals were shown to be as effective as PCNB, but they displayed varying degrees of phytotoxic effects on the plant (McIntosh 1973;1976). Later trials evaluated foliar applied chemical treatments, and found 2,4-dichlorophenoxyacetic acid (2,4-D) and 3,5-dichlorophenoxyacetic acid (3,5-D) to be highly effective in controlling disease in glasshouse trials. However both materials produced significant phytotoxic effects on treated plants, including reduced yield, an increased numbers of small tubers, and tuber deformation. Further to these phytotoxic effects, control of common scab by 3,5-D dropped dramatically from glasshouse trials (90% reduction in disease severity) to field trials (30% reduction). As such, it was not considered to be a viable control method by the authors (McIntosh et al. 1981; 1982).

The mechanism by which 2,4-D, a synthetic auxin and potent herbicide, controls common scab was not determined by McIntosh and his colleagues. Effective levels of 2,4-D did not affect the growth of the pathogen (McIntosh et al. 1981; Tegg et al.

2008), or its ability to produce thaxtomin A (Tegg et al. 2008). Studies by Tegg et al. (2008; 2012) demonstrated a strong relationship between disease levels and the tolerance of tuber tissues to thaxtomin induced toxicity, in plants that have been treated with 2,4-D. Furthermore, Tegg et al. (2012) found that a range of other foliar applied chemicals which also reduce common scab similarly increase tuber

thaxtomin tolerance. They showed a significant negative correlation between the extent of disease expressed and the reduction in necrosis following thaxtomin treatment in tubers harvested from treated plants. This suggested 2,4-D and these other materials may operate by reducing sensitivity of potato tubers to damage by thaxtomin, and by this reducing the invasive capacity of the pathogen.

In attempts to optimise the application of 3,5-D to obtain the greatest disease suppression, McIntosh et al. (1981) found that earlier applications of 3,5-D in the plants development increased the efficacy of disease control, but it also increased the material‘s phytotoxic effects on the potato plant, further decreasing yield and tuber quality. Through the use of strategically applied irrigation treatments, which were known to suppress common scab, Lapwood et al. (1973) were able to confirm that tubers were only susceptible to infection by pathogenic Streptomyces spp. for a defined period, coinciding with tuber elongation, and presence of immature (non- suberised) lenticels (Lapwood et al. 1973; Khatri et al. 2011; Adams 1975). This

occurs approximately four to six weeks after tuber initiation, when stomata transform into lenticels (Lapwood & Hering 1970). Initial Australian studies examining the use of 2,4-D and other chemicals for the control of common scab targeted this critical infection period with treatment applications, scheduling treatments for 2 weeks after tuber initiation (Tegg 2006; Tegg et al. 2008). In these studies, tuber initiation was estimated by visual assessment of plants for hook development on underground stolons, and given tuber initiation for any potato plant is asynchronous with tubers initiating periodically during plant development, accurate timing of treatments to cover this ―infection window‖ was difficult. There was a possibility that treatments may have been applied after some (or perhaps many) developing tubers had already entered the critical infection period, and thus would have failed to protect these tubers from infection. Furthermore, timing of treatment to coincide with the period of susceptibility does not allow for variation in the rate or efficiency in the translocation of the applied chemicals to the tubers. Burrell (Burrell 1982) found that after 1 day only 0.3% of 2,4-D applied to foliage had been translocated to tubers, and after 4 days it increased to only 2-3%. As these treatments have no biocidal effect against the pathogen, it is not believed that they have any effect on disease development once infection has occurred.

Previous studies (Tegg 2006; Tegg et al. 2008) used multiple applications staggered throughout the infection period in order to increase the duration of treatment to protect the tuber for the entire period it is susceptible to infection. Tegg et al. (2008) found that multiple applications provided greater control against common scab than single applications, when the initial application was targeted to the beginning of the infection window.

This chapter describes studies that aimed to determine the optimal timing for application of 2,4-D foliar treatments to provide the greatest common scab control; to determine the effect of these treatments on the levels of 2,4-D in tubers at harvest; and to further examine the effect of single and multiple treatments on disease control when applied at various stages of plant development.