Efficacy and crop tolerance of Stamina (pyraclostrobin) and Flite (triticonazole) seed treatment formulations against Fusarium, Pythium and Rhizoctonia soilborne diseases of maize

Abstract

Maize (Zea mays L.) is a cereal crop grown throughout the world. It plays an important role in the diet of millions of African people due to its high yields per hectare, its ease of cultivation and adaptability to different areas, its versatile food uses and storage characteristics (Asiedu, 1989). Maize is a staple crop in Southern Africa where it accounts for 70% of total human intake of calories (Martin et al., 2000). Thus it is essential that maize can be sustainably produced in South Africa and that maize seeds are of the highest possible quality.

Fungi rank as the second biggest cause of deterioration and loss of maize (Ominski et al., 1994). At the very early stages of seedling development, maize seedlings are attacked by fungi such as Pythium, Fusarium and Rhizoctonia spp., which cause severe diseases, including pre-emergence damping-off, which lead to yield losses (Dodd & White, 1999). These diseases can be effectively controlled by applying fungicidal seed treatments (Peltier et al., 2010). However, these seed treatments should be tested to ensure that they provide an acceptable level of control against the pathogens and that they do not have any negative effects on the germination and vigour of the maize seed.

In Chapter 3 of this dissertation, three important fungal genera, namely Pythium, Fusarium and Rhizoctonia spp., were isolated from diseased maize plant samples and soil. The beet seed baiting method was used for Rhizoctonia sp. and the citrus leaf disk baiting method for Pythium sp. Fusarium sp. was isolated by means of serial dilution on a selective medium. The selective media used were agar containing chlorotetracycline hydrochloride and streptomycin sulfate for Rhizoctonia, pimaricin and vancomycin, PARP (pimaricin + ampicillin + rifampicin + pentachloronitrobenzene (PCNB) agar) for Pythium sp. and Rose Bengal Glyceraldehyde Urea (RBGU) for Fusarium sp. These fungal isolates, as well as some isolates revivedfrom the University of Pretoria’s culture collection and obtained from the Agricultural Research Council (ARC-PPRI), were used for pathogenicity trials conducted on maize in the between-paper method (BP), and in six-celled plastic seedling trays in the greenhouse (described in Chapter 5).

In order to test the efficacy of Stamina, Flite and Celest® XL for controlling Pythium spp., Fusarium spp. and Rhizoctonia spp. in vitro, each of the three fungicides was added to PDA at concentrations of 1, 2 and 3ppm. In order to mirror the treatments used in other experiments, a combination of Stamina and Flite was also incorporatedinto PDA at concentrations of 1, 2 and 3ppm each. A 5mm2 block of each of the fungi was plated onto the centre of the media and incubated at 25C. The diameter of the fugal growth was measured at regular intervals depending on the rate of growth of the fungus. It was found that Celest® XL was very effective in controlling all three of these pathogens in vitro, confirming research done by Govender (2005), who found that Celest® XL effectively controlled these pathogens on maize. The combination of Stamina and Flite also controlled these pathogens although to a lesser extent. Research done by BASF in 2008 showed that Stamina is able to control Pythium, Fusarium and Rhizoctonia spp. Pyraclostrobin (the active ingredient of Stamina) has also been found to effectively control all three of these pathogens in numerous in vitro and in vivo experiments (Broders et al., 2007; Peltier et al., 2010; Solorzano & Malvick, 2011).

In Chapter 4 of this dissertation, the effect of three different fungicides (Stamina, Flite and Celest® XL) on the germination and vigour of two Zea mays cultivars (Monsanto DKC78-15B and PANNAR 6Q308B) was assessed. This was achieved by carrying out a standard germination test, a cold soil test, short accelerated ageing and long-term storage tests according to the guidelines of the International Seed Testing Association (ISTA, 2012). It was found that none of the fungicides had a detrimental effect on either seed germination or vigour and no phytotoxic effects were observed. The combination of Stamina and Flite treatment also led to an increased percentage germination after the cold soil test when compared to the untreated control. This confirms the research of Govender (2005), who showed that Celest® XL had no negative effects on the germination or vigour of maize, and BASF (2008), which showed that Stamina could even lead to increased germination and an increased yield of maize under cold conditions when compared to an untreated control. Bradley et al. (2001) found that fungicide seed treatments do not affect the vigour and viability of maize seeds. Seeds treated with fludioxonil also showed an increased radicle length in some cases (Munkvold & O’Mara, 2002). Increased radicle length could indicate increased vigour of the seeds (Matthews & Khajeh-Hosseini,2006).