Effect of seed treatment with milk powder and mustard flour in control of common bunt (Tilletia tritici) in wheat and stem smut (Urocystis occulta) in rye

Anders Borgen
Holmemosevej 7, 4780 Stege, Denmark, e-mail: anders[at]ecoweb.dk

Lars Kristensen
Agroecology, Royal Vet- and Agricultural University, Agrovej 10, 2630 Tåstrup, Denmark

ABSTRACT

In field trials mustard flour was able to control seed borne infection by common bunt (Tilletia tritici) in wheat without decreasing the germination vigour of the treated seeds. Full control of common bunt by coating the seeds with milk powder could only be achieved at doses which reduced germination vigour of the seeds. Mustard flour can be recommended as a seed treatment in organic agriculture while a treatment based on milk powder should be developed in combination with biological control. Both milk powder and mustard flour can be used to control stem smut in rye (Urocystis occulta).

INTRODUCTION

Common bunt (Tilletia tritici syn. T.caries) in wheat is one of the plant diseases most intensively controlled with fungicides. Almost all conventional seed lots are today treated with synthetic fungicides and the disease is a major threat to organic wheat production where these seed treatments are not permitted (Borgen, 2000; Nielsen et al., 1998). In recent years, new means of treatments have been developed adaptable to organic farming. Among these are milk powder (Becker, 1992; Becker & Weltzien, 1993; Borgen & Davanlou, 2000; Borgen & Kristensen, 2000; Borgen et al., 1995; Heyden, 1993, 1997; ICARDA, 1996, 1997; Nielsen, 1998; Nordin, 1991; Plakholm, Plakholm & Söllinger, 2000; 1993; Tränkner, 1993, 1996; Winter et al., 1997), mustard flour (Spiess & Dutschke, 1991) and acetic acid (Borgen & Nielsen, 2001).

Stem smut (Urocystis occulta) in rye has achieved little attention as a seed borne disease in the post war period since the pathogen is easily controlled by synthetic pesticides. Our experience in Denmark is that the disease is present in the cropping system when seed lots are maintained through several years without seed treatments, but the diseases rarely develops into a yield depressing frequency. However, some times it does, and a program for organic seed production should therefore include possibilities to control the disease in cases of propagation.

High doses of seed treatments in some cases reduces germination vigour of the seeds resulting in a poor or delayed germination in the field. In organic agriculture the phase of establishment of the crop is very important for the yield, since the competition with weeds will depend on a quick and uniform crop stand. Special focus should therefore be taken when working with seed treatments for organic agriculture. The increased cropping of wheat in Europe in de past decades has led to an increased significance perennial survival of inoculum of Tilletia tritici in the soil (Borgen 2000b). The aim of the present study is to investigate the potential of milk powder and mustard flour as a seed treatment against common bunt in wheat and stem smut in rye with further focus on the side effect of the treatment on the seed germination vigour.

MATERIALS AND METHODS

Field trials were conducted at Højbakkegård on Zealand, Denmark in the period 1995-1997. The effect of different doses of milk powder and mustard flour were tested by applying increasing doses to seeds of wheat. The seeds were contaminated with 5 g spores of either Tilletia tritici or Urocystis occulta per kg seeds, which resulted in a contamination between 1.7 to 2.0 mill spores per gram seeds when tested by the ISDA haemocytometer method (Keitreiber, 1984). After treatment the seeds were stored at 5 C?. Samples were taken out for field test 2-6 days after seed treatment. Germination tests were conducted 1-3 month later.

Germination tests in lab were done as a cold sand test in plastic plates containing 1.5 kg sand with water (65ml H2O/kg quartz sand). Sowing depth was 1.5 cm and temperature was 10 C?. The number of emergent seedlings were counted every day for 5 days after first emergence. The number of replicates were 3-4 and the seed vigour is expressed as the time for 50% emergence.

In the field trial with seed borne infection with common bunt in 1995 and the with stem smut in 1997, plots were sown in 6 m2 plots in 4 replicates. In the field trials with common bunt 1996-97, treatments were sown in rows of each 1.25 m in 10 replicates. Total number of diagnosed plants in these trials was 1-2000 on average in each treatment with common bunt and 550 with stem smut. After heading the ratio of infected ears of wheat and infected plants of rye were counted based on visible macro-symptoms.

The field trial with soil borne infection was made in a field where a highly infected wheat field was grown in 1996. In 1997 a barley field was grown and after this, the experiment was established in winter wheat plots 8 x 1? m in 4 replicates with seeds without any spores of common bunt. All plants in all plots were diagnosed for infection.

Data of diseased plants and germination speed was analysed by a generalised linear model (PROC GENMOD in SAS ver. 6.12).

RESULTS

The effect of increasing doses of mustard flour on infection of wheat by common bunt is shown in Figure 1. In 1996 and 1997 the highest doses of 33 and 43 g/kg significantly reduced germination vigour. In both years it was possible to achieve a reduction not significantly different from 100% at lower doses without any decrease in germination vigour. This is consistent with previous studies by Spiess and Dutscke (1991), even their investigation of the side effect on germination vigour was only recorded by the seedling emergence in the field. This indicates that mustard flour can be used as an effective fungicide against seed borne infection by common bunt.

Three Brassica species (brown mustard (Brassica juncea), oriental mustard (Brassica juncea var.orientalis) and oil seed rape (Brassica napus)) were tested against the standard mustard flour made from yellow mustard (Brassica hirta syn. Sinapis alba). All species were significantly less effective than yellow mustard (data not presented). A range of derivates from Brassica glocosinolates were tested as pure chemicals. Some effects were seen from derivates from Sinapis alba, while no or less effect were seen from derivates from the other species (data not presented).

The effect of increasing doses of milk powder on infection of wheat by common bunt is shown in Figure 2. In 1996 the maximum control not significantly different from 100% was achieved at the dose of 43 g/kg and 1997 this was achieved at 80 g/kg. In both years a significant reduction in germination vigour was recorded at this dose of maximum control and all higher doses. This shows that milk powder is not an optimal measure in control of seed borne infection by common bunt as proposed by previous studies (Becker & Weltzien, 1993; ICARDA, 1996, 1997; Plakholm, 1993; Plakholm & Söllinger, 2000; Tränkner, 1993, 1996; Winter et al., 1997).

Becker & Weltzien (1993) has shown that the mechanism of action of the milk powder is like to be explained by micro-organisms using the milk powder as a nutrient source. Rabien (1928) has shown that the pathogen is restrained in development in the presence of sugar as an effect of competition with other organisms for oxygen.

Milk powder was compared with other organic substances as shown in Table 1. The uniformity of the coating of the substances on the seeds was visible, and the differences in effect between the substances is likely to be a combination of both chemical and physical factors. The side effect on seed germination at the high doses of milk powder also may be a result of the decreased availability of oxygen and maybe other factors needed for germination. Milk powder used alone in combination with natural occurring saprophytes in the soil or seed surface can not be used as an fully effective control measure without a negative side effect on germination, and this is also the case with some formulations of biological control. Recent studies of biological control of common bunt shows that limited doses of milk powder (20 g/kg) in combination with biological control agents have a synergistic effect and can give an effective control without reducing the germination vigour (Borgen & Davanlou 2000). This combination is believed to be the future for milk powder and equivalent agents in the control of common bunt.

Table 1: Frequency of common bunt as affected by seed treatment with 30 g/kg of different compounds. Disease rate tested in field trials; germination speed tested in lab.

Milk powder and mustard has been tested against natural soil born infection by common bunt, and milk powder reduced the frequency of diseased plants by 91% (p<0.001) (Borgen & Kristensen, 2000) while mustard had no significant effect. However, this is based on only one year of field experiments and contradicts conventional conclusions from experiments with artificially contaminated soils that soil borne infection can only be controlled by systemic pesticides. Further studies should clarify the potential of milk powder in control of soil borne infection by common bunt.

Table 2: Effect of milk powder and mustard flour on the infection of stem smut of rye (Urocystis occulta)

Milk powder and mustard flour was tested against stem smut in rye (Urocystis occulta) resulting in a reduction of infection by >90%. The treatments were tested in only one dose, and this dose did not affect the germination vigour of the seeds. Whether a higher dose could increase the effect without side effect on germination was not tested. The experiment indicates that both agents may be used as a seed treatment against the pathogen.

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Figure1: Field trials with common bunt as affected by increasing doses of mustard flour. Bars indicating 95% confidence intervals.

Figure 2: Two years of field trials with common bunt as affected by seed coating with milk powder. Bars indicating 95% confidence intervals.