Damping-off of sugar beet in Finland . 11 . Disease control

In pot and field experiments carried out in 1979-1981, the systemic funqicide hymexazol prevented satisfactorily soil borne damping-off of sugar beet caused mainly by the fungus Pythium debaryanum auct. non Hesse. The results with the combination hymexazol + thiram were still better. This treatment gave very good protection against the disease up to about two to three weeks after emergence, increased the yield on the average by 5-10 % and produced considerably thicker and denser stands. Thereafter a large number of beets may have become infected, but no great damage was caused as only few died. Band spraying at emergence using hymexazol with a large amount of water as well as spraying into the seed furrow prevented the outbreak of the disease almost completely. Liming had little effect on damping-off. Introduction In modern sugar beet cultivation, the method of sowing to stand has become common. According to DUNNING (1970), the fungicide treatment of sugar beet seed is commonly applied in most countries and cannot be dispensed with, because damping-off is considered one of the most significant diseases of beet. In Europe during 1968-1970, the greatest losses due to damping-off occurred in Poland, Romania, Czechoslovakia and Hungary. In Ireland, too, the disease was reported to be severe (DUNNING 1970). In 1979-1980, the main disease-causing factors at the seedling stage in 16 European countries were in the decreasing order of frequency: Phoma (13), Pythium (13), Aphanomyces (8), Rhizoctonia (6), Fusarium (3), Alternaria (2) (DUNNING & HEIJBROEK 1981). In controlling the seed borne damping-off of sugar beet, seed dressing with mercurial compounds or thiram has been predominating (GATES & HULL 1954, GATES 1959, NOLLE 1960, LUDECKE & WINNER 1963, LINNASALMI 1970, MÖLLERSTRÖM 1974). However, maneb and mancozeb have


Introduction
In modern sugar beet cultivation, the method of sowing to stand has become common.According to DUNNING (1970), the fungicide treatment of sugar beet seed is commonly applied in most countries and cannot be dispensed with, because damping-off is considered one of the most signific- ant diseases of beet.In Europe during 1968Europe during -1970, , the greatest losses due to damping-off occurred in Poland, Romania, Czechoslovakia and Hungary.In Ireland, too, the disease was reported to be severe (DUNNING 1970). In 1979-1980, the main disease-causing factors at the seedling stage in 16 European countries were in the decreasing order of frequency: Phoma (13), Pythium (13), Aphanomyces (8), Rhizoctonia (6), Fusarium (3), Alternaria (2) (DUNNING & HEIJBROEK 1981).
In controlling the seed borne damping-off of sugar beet, seed dressing with mercurial compounds or thiram has been predominating (GATES & HULL 1954, GATES 1959, NOLLE 1960, LUDECKE & WINNER 1963, LIN- NASALMI 1970, MÖLLERSTRÖM 1974).However, maneb and mancozeb have also been reported to control damping-off (DARPOUX et al. 1966, KOCH 1979, HRUBESH & WIESER 1978).In recent years, seed treatment with iprodione, metalaxyl or hymexazol has proved effective against soil borne pathogens (DUNNING & HEIJBROEK 1981).As early as in 1952, HILLS and LEACH found certain soil-row spray treatments to be effective and superior to standard seed treatments.SCHULTZE and BOHLE (1976) also achieved promising results with this method using fenaminosulf as soil fungicide. VESELY (1978) has been able to control damping-off of sugar beet biologi- cally using oospores of the fungus Pythium oligandrum Drechs.as seed dressor.
In Finland, the damping-off of sugar beet is firstly caused by the soil borne fungus Pythium deharyanum auct.non Hesse and secondly by diffe- rent species of Fusarium (VESTBERG et al. 1982).The aim of this study was to find out the most effective ways of controlling damping-off of sugar beet by means of seed dressing, spraying or soil treatment and preventive control by liming.Some preliminary biological control experiments have also been carried out.The investigation were conducted during 1979-1981 with the cooperation of the Department of Plant Pathology, University of Helsinki, and the Finnish Sugar Beet Research Centre.

Materials and methods
The following pot experiments were carried out to control damping-off: seed dressing, spraying treatments, application of fungicide to the seed furrow and surface soil, liming and biological control experiments.In the experiments, naturally contaminated soil from the sugar beet field at Viikki Experimental Farm was used.The sugar beet seed used was Monohill.Fungicide spraying was carried out with a hand spray either at emergence and/or at different intervals after emergence.The amounts of water used ranged from 0.1 to 1.5 1/m 2 .In the liming experiments, hydrated fine ground lime was mixed into the pot soil.In one experiment hymexazol was mixed into the soil surface, about 5 cm deep.
All the pot experiments were carried out in a greenhouse at +lB -+2O°C.Two experiments were carried out alongside at lower temperature, +8 - + 10°C.
In the pot experiments, emergence was followd and healthy and diseased seedlings were counted at regular intervals after emergence until there were no symptoms of damping-off.

Results
In a premilinary experiment against damping-off, captafol proved the most effective at emergence, but at the end of the experiment propamocarb seed treatment gave the greatest number of healthy seedlings.Metalaxyl + thiram (0.25+4.0 g) 95 35 Propamocarb (18.8) 84 68 Thiram (4.0 g) 89 10 The seedlings were sprayed at emergence and at different stages thereafter, but no additional effect was observed.
In a pot experiment in 1980, propamocarb and hymexazol were used as seed dressors at two temperatures.The temperature clearly affected the extent of infection and the control result.At low temperatures, only moderate outbreads of damping-off occurred and all the seed dressors used proved very effective as compared with untreated plots.At high temperatures, there were severe outbreaks of damping-off and only hymexazol gave good results.Propamocarb + tiophanatmethyl (10.5+1.0 g) 95 76 Propamocarb + benomyl (10.5+1.0 g) 91 21 Thiram (4.0 g) 85 3 Hymexazol alone at high temperatures gave better results than hymexazol combined with tiophanatmethyl or benomyl.In 1980, a seed dressing and spraying experiment was carried out in a greenhouse at high and low temperatures.With regard to temperature, the results were similar to those in the seed dressing experiment of the same year reported above.Spraying at emergence did not improve results at low temperatures.At high tempera- 4 433 tures, only hymexazol and hymexazol 4-tiophanatmethyl were effective, particularly when sprayed.Spraying at emergence with greater amounts of water gave better results.Hymexazol (10.5 g a.i./kg seed) No spraying 83 10 Hymexazol (10.5 g) 86 55 Hymexazol (10.5 g) (5.25 g, 1.5 1) 88 83 Hymexazol (10.5 g) + tiophanatmethyl (1.0 g) No spraying 84 17 Hymexazol (10.5 g) + tiophanatmethyl (1.0 g) (1.05+0.11g, 0.3 1) 86 61 Hymexazol (10.5 g) + tiophanatmethyl (1.0 g) (5.25+0.55g, 1.5 1) 85 98 Propamocarb (18.8

Thiram
No spraying 89 8 In another pot experiment in 1980, variable amounts of hymexazol (1.4- 14.0 g/kg seed) were tested using both seedling spraying and soil treatment (Table 1).Due to the fact that untreated seed emerged well (90 %), no difference was found with regard to emergence between the amounts used for seed dressing.At the end of the experiment, only 9 % of the untreated seedlings were healthy, and even seed dressing (3-14 % healthy seedlings) proved ineffective against damping-off in this experiment.Spraying espe- cially with great amounts of water, proved somewhat effective, but the best results were obtained by mixing the fungicides into the soil.This treatment gave 75-92 % healthy seedlings (Table 1).
A hymexazol experiment in 1981 showed that only when the amount of seed disinfectant used rached 7.7 g/kg seed did not control result improve significantly (Table 2).In another experiment of the same year, hymexazol was applied to the seed furrow immediately before sowing (Table 3).The damping-offpercentage was 70 % with untreated seed, but only 9 % when as little as 0.005 g/m 2 was applied to the seed furrow.
In greenhouse experiments, biological control of damping-off has been studied using oospores of the fungus P. oligandrum as seed dressing.In an experiment in 1980, there was 97 % damping-off in untreated plots, while in treatments with P. oligandrum the result was 70-99 %.There was a statisti- cally significantly greater number of healthy seedlings only with the combination P. oligandrum + benomyl (Table 4).In a similar trial in 1981, there were no statistically significant differences in the health of the seedlings.
In the greenhouse liming experiment, the percentage of healthy seedlings rose from 29 to 59 % with pH rising from 5.2 to 6.6 (Table 5).The effect of liming on emergence was not equally significant: 3 and 6 g/1 soil increased emergence, while 9 g/1 soil decreased emergence.Liming was also discovered to improve the effect of fungicides (Table 5).Table 4.The effect of seed treatment with oospores of Pythium oligandrum and some fungicides against damping-off of sugar beet.Table 5.The effect of liming and seed treatment on emergence and percentage of healthy sugar beet plants at the and of the experiment.Greenhouse experiment in 1980.

Liming pH
No limp application 5.2 Hydrated fine ground lime 3 g/1 soil 6.1

Materials and methods
The locations of the field experiments were chosen on the basis of earlier experiments which showed the most severely infected fields owned by sugar beet farmers.Seed dressing tests and seed dressing 4-spraying tests were carried out yearly.Liming tests were conducted in 1980 and 1981.In 1981, the application of fungicide to the seed furrow was studied.In addition to chemical tests, biological fungicides were used in two experiments.
The Sugar Beet Research Centre was responsible for the practical arrange- " 6 " " 6.5 " g " " g g

Liming
No lime 3 g/l soil 6 g/l soil 9 g/l soil Healthy Healthy Band spraying was done with a propane spray and a large amount of water.In 1979, 3000 1 of water/net ha was used and the spraying band was about 10 cm, in 1980 and 1981, 2000 1/net ha and 5 cm, respectively.Hydrated fine ground lime was used to raise the pH level.In 1980, 3 and 6 tons/ha was spread onto the fields after sowing, whereas in the 1981 liming test it was spread before sowing and tilled in.In 1981, an attempt was made to raise the pH to 7.5.The amount needed to reach this level was called the ''normal liming amount".Also 1.5 times this amount was used.In field experiments, Pythium oligandrum biopreparation and Eokomit compost preparation were used for biological control.Eokomit is a mixture of certain bacteria living in the soil, mostly Azotobacter chroococcum Beijerinck., A. beijerinckii Lipman, Bacillus suhtilis and B. stearothermophilus Donk.
In 1979 and 1980, the experimental plots consisted of only 1 or 2 beet rows, 2x5 m, 2x16 m or Ix3o m, depending on the trial.In 1981, the plots consisted of 5 20-m rows, except for the plots in the liming test which were 10 mXIO m.All the experiments were inspected twice and the healthy and infected seedlings counted: the first time about 10 days after emergence with the seedlings at the cotyledon stage, the second time about 10-14 days later when the seedlings had 2-4 pairs of true leaves.In 1979, the number of seedlings along a length of 6 m per plot were counted, in 1980 along 10 m and in 1981 along 12 m per plot.In 1979, the yield was not examined after the damping-off stage, whereas in [1980][1981] the yield, number of beet roots per ha and sugar percentage were determined in the autumn.The samples were taken along a length of 15 m per plot in 1980 and 20 m per plot in 1981, but not the same place damping-off samples were taken from in the early summer.
Weather conditions 1979-1982   The mean mothly weather conditions over three years are shown in Table 6.In 1979, May and June, which are important months with regard to damping-off, exceeded normal temperatures.In 1980, May was rather cool and June very warm, in 1981 vice versa.On the whole the growing season during [1979][1980] was warmer than normally and in 1981 somewhat colder than normally.
In 1979, May and June were drier than normally, whereas in July, August and September rainfall was heavy.In 1980, July was rather dry, in August it rained heavily, whereas rainfall was normal during the other months.In 1981, rainfall was heavy in July and August, whereas June and September were relatively dry Table 6).Table 6.Monthly mean temperature and rainfall during May -September 1979-1981 expressed as differences from ''normal" (1931-1961) values.Averages from the following observation stations: Helsinki Kaisaniemi, Hattula Leteensuo, Turku Airport and Kokemäki Peipohja.

Results
The choice of fungicides for the field experiments was based on the results of the pot experiments.In 1979, the early summer was warm and favourable for the disease.The seed dressing experiment in four locations showed on the average 44 % of damping-off in the untreated seed and 44-60 % in treated seed.No difference could be found in the number of healthy seedlings per row meter after the damping-off stage between untreated and treated seed (Table 7).
The average results of eleven experiments showed different seed and spraying treatments to have very little effect on damping-off (Table 8).After the damping-off stage there was an increase in the number of healthy seedlings per row meter of 0.1-0.5 after spraying, as compared with the untreated plot.This was not, however, statistically significant.
In 1980, there were serious outbreaks of damping-off, as was the case in 1979.In the seed dressing experiment there was 67 % damping-off in the untreated plot (Table 9).The pelleted seed showed the lowest percentage of  damping-off, only 13 %.When hymexazol was increased from 3.5 to 14.0 grams per kg seed, the percentage of damping-off fell from 57 to 43 %.The dipping of seed into a propamocarb solution did not lower the percentage of damping-off, and P. oligandrum had no protective effect in this respect.The number of healthy seedlings per row meter at the end of June was highest with pelleted seed, 4.1, and lowest with P. oligandrum treatment, 1.4.In the untreated plot there were 1.8 healthy seedlings.Seed dressing with hymex- azol, 3.5-14.0g/kg seed, led to an increase of 0.8-1.7 in seedlings/row meter as compared with the untreated seed.The yield in the experiment was quite high, 35 tons/ha on the average.All the treatments except for P. oligandrum preparate increased the yield.The dipping of seed into a propamocarb solution increased the yield by 6.2 tons and the treatment of seed with hymexazol by 4.1-4.7 tons.In the autumn, untreated plots produced 61200 beets/ha on the average, whereas plots treated with hymexazol produced about 80000 beets/ha.All seed treatments gave somewhat higher sugar contents than the untreated seed, the highest increase being with prop- amocarb, 0.56 %, and the lowest with hymexazol, 0.04 %.
In 1980, an experiment was made to study the effect of hymexazol and propamocarb using spraying.Hymexazol was used in two concentrations, 0.5 % and 0.1 %.The spraying of seedlings at emergence gave good control results with the 0.5 % suspension of hymexazol alone and hymexazol in combination with thiram (Table 10).The damping-off percentage fell to 0-12 % as compared with the 40 % in the control, the increase in yield was 1.9-4.2tons/ha and in the autumn there was an increase of 7100-17400 beets/ha.Propamocarb also increased the yield and controlled damping-off to some extent.In the experiment with 0.1 % of hymexazol there was 78 % of damping-off (Table 10).Seed dressing with hymexazol or propamocarb did not produce healthier stands than untreated seed, whereas with hymexazol + thiram there was only 45 % of damping off.Propamocarb spraying had no effect, yet spraying with hymexazol + thiram decreased damping-off to 17 %.The amount of beet produced was about the same either with spraying or seed dressing.In this experiment, treated seed have higher sugar contents than untreated.Seed dressing with the combimation hymexazol + thiram gave a significantly greater yield.
In 1981, it was colder and rainier during the early stage of seedling development than in 1979 and 1980, and damping-off occurred less frequently than in 1979-1980.That year, two seed dressing experiments were made.In one experiment the control treatment was dressed with thiram and the damping-off percentage was 58 % (Table 11).Using hymexazol, 14.7 g/kg seed, the percentage was much lower and the yield on the average 2.7 tons higher per hectare than in the untreated plot, which produced a yield of 23.7 tons/ha.The effect of seed treatment on the sugar content varied.When the seed was pelleted (Table 12), the effect of seed dressing on the number of healthy seedlings was the same.The increase in the yield was, however, only 0.2-1.5 tons/ha.
In the spraying experiment in 1981, hymexazol showed a good effect on the health of the seedlings.Thiram or propamocarb + benomyl did not give  11.The effect of seed treatment on the damping-off frequency, number of healthy beet plants per row meter in final stands, number of beet roots per ha, yield and sugar content as means of 4 field experiments in 1981.
Table 12.The effect of seed pelleting with fungicides on the damping-off frequency, number of healthy beet plants per row meter in final stands, number of beet roots per ha, yield and sugar content as means of 4 field experiments in 1981.
the same results (Table 13).Spraying with hymexazol into the seed furrow showed promising (Table 14), although there were some technical problems.The advantage of this method is the fact that only small amounts of fungicide are needed.When the damping-off percentage in the thiram control was 65 %, with hymexazol and thiram seed dressing it was only 31 %, and when spraying hymexazol 0.14 kg/ha into the seed furrow, only 13 %.With 0.84 kg/ha there were no symprtoms of damping-off whatsoever.
In a liming experiment in 1980, limig had little effect on damping-off (Table 15).With 6 tons of lime/ha the number of seedlings per row meter Table 13.The effect of seed dressing with the combination hymexazol 4-thiram and spraying on the percentage of damping-off, number of healthy plants/row meter in final stands, numberof beet roots/ha, yield and sugar content as means of 4 field experiments in 1981.
Table 14.The effect of seed treatment with the combination hymexazol 4-thiram sprayed into the seed furrow on the percentage of damping-offand number of healthy sugar beet plants/row meter in final stands.Field experiment in 1981.after the damping-offstage at the end of June increased on the average by 0.2, and the percentage of damping-off fell from 51 to 42 %.
In the liming experiment at the end of June 1981, a pH of 6.9 was measured after the ''normal liming amount" and after 1.5 times the amount, pH still remained at 6.9, the plots without lime being of pH 6.0.When the damping-offpercentage was 69 % without liming, it was 55 % on the average after the larger amount of lime.Liming gave a 1 % increase in the yield, whereas the sugar content fell by 0.22-0.30%.The compost preparete Eokomit was used together with lime, but it had no additional effect on the incidence of damping-off (Table 16).Table 15.The effect of lime application and seed treatment on the percentage of damping-off and number of healthy sugar beet plants/per row meter in final stands as means of 4 field experiments in 1980.
Table 16.The effect of lime application on the percentage of damping-off, number of healthy beet plants/ row meter in final stands, number of beet roots/ha, yield and sugar content as means of 3 field experiments in 1981.
11 Hydrated fine ground lime application to reach pH 7.5

Discussion
Out of several preparations tested, the systemic fungicide hymexazol proved the most effective seed dressor against damping-off of sugar beet.Mercurial compounds and thiram had very little effect.These results are in agreement with those of DARPOUX et al. (1965) and KOCH (1979).The systemic fungicide propamocarb which is effective against Pythium species (ANON: 1978) seemed to be to some extent effective against damping-off under certain conditions, i.e. low temperature and moderate outbreaks of damping-off.In a field experiment, dipping of the seed in a solution of propamocarb gave the gratest yield, although the incidence of damping-off was as high as in the control.In this case, therefore, the positive effect of the treatment seems to be due to pre-germination rather than to the fungicide.With regard to the effect of temperature, two greenhouse experiments at 8 °C and 18°C showed the response to seed dressings to increase with decreasing temperature, a fact that is in agreement with the findings of GATES and HULL (1954).At B°C, the treatment of seed even with mercurial compounds was effective against soil borne damping-off, but at 18°C the effect was negligible.However, under field conditions, only hymexazol and especially hymexazol 4-thiram proved effective seed dressors.Hymexazol is reported to be effective against a range of soil borne diseases, especially those caused by Pythium, Fusarium and Aphanomyces (ANON. 1971).
The treatment of the seed with hymexazol + thiram gave, in most cases, very healthy stands and in 1982 this treatment came into commercial use in Finland.In some cases, both in pot and field trials, the incidence of the disease was approximately as high as with untreated seed, but it attacked at a later stage and was not as disastrous as in the control.Chronic damping-off did occur, but the plants recovered pretty well.Hymexazol is not only an effective fungicide against Pythium and Fusarium, but also an effective plant growth promotor (KUKALENKO & VOLODKOVICH 1979).Both pot and field experiments showed higher amounts clearly to improve the effect of hymex- azol.The optimum amount would be about 10 g/kg seed combined with thiram, 4 g.The use of thiram together with hymexazol improved the health of beet stands more than did hymexazol alone.The same was observed by HRUBESH and WIESER (1978) who also recommed mancozeb in this respect.
The effects of soil treatment against damping-off of sugar beet have been investigated in both pot and field trials.Band spraying with hymexazol and thiram, using very great amounts of water (up to 20000 1/net ha), gave almost a 100 % control of the disease.However, the favourable effect of such treatments on the yield in comparison with seed dressing alone was very small except in some districts with very severe outbreaks of the disease.Moreover, spraying treatment requires large amouts of the fungicide to be effective, which is certainly not economically profitable for the farmer.The application of the fungicide, using small amouts, to the seed furrow in connection with sowing gave very good results, but the method needs further development.In conclusion, the treatment of seed with hymexazol + thiram seemed, however, to provide a good protection against the disease.
Preventive control of damping-off by liming gave promising results in a pot experiment.This experiment also showed the response to different seed dressings to rise with increasing soil acidity.In field experiments during 1980-1981, liming had not clearly positive effect on the incidence of damp- ing-off.This, presumably, was due to the fact that the pH did not rise high enough to prevent damping-off caused mainly by Pythium.
P. oligandrum is reported to be an effective seed dressor against damping- off caused by Pythium (VESELY 1978(VESELY , 1979)).However, in this study the Czechoslovakian biological preparation acted more as a pathogen than as an antagonist against Pythium deharyanum.Therefore, a successful biological control of damping-off would most probably require organisms isolated from Finnish soils.The biological control of damping-off as a whole needs further investigation.

Table 1 .
The effect of seed and soil treatment on emergence and percentage of healthy sugar beet plants at the end of the experiment.Greenhouse experiment in 1980.

Table 2 .
The effect of seed treatment with hymexazol and thiram on emergence and percentage of healthy beet plants at the end of the experiment.Greenhouse experiment in 1981.

Table 3 .
The effect of hymexazol seed furrow application on emergence and percentage of damping-off of sugar beet.Greenhouse experiment in1981.
Emer-the expc-Emer-the expo-Enict-the expe- related to the experiments during the growing season.Seed dressing was carried out at the Sugar Beet Research Centre as near to sowing time as possible.The seed used was naked Monohill.In an experiment in 1980, pelleted seed was used in one treatment, whereas in 1981 pelleted seed was used in all treatments. ments

Table 7 .
The effect of seed treatment on the damping-off frequencyand number ofhealthy beet plants per row meter in final stands as means of experiment in 4 locations in1979.

Table 8 .
The effect of seed treatment and plant spraying on the percentage of damping-off and number of healthy sugar beet plants/row meter in final stands as means of 11 field experiments in 1979.

Table 9 .
The effect of seed treatment on the damping-off frequency, number of healthy beets per row meter in final stands, number of beet roots per ha, yield and sugar content as means of 4 field experiments in 1980.