Maataloustieteellinen Aikakauskirja Vol. 56: 283—290, 1984 Damping-off of sugar beet in Finland. 111 Effect of temperature and disease forecasting MAURITZ VESTBERG Department of Plant Pathology, University of Helsinki, SF-00710 HELSINKI 71, Finland Abstract. The effect of temperature on the severity of damping-off of sugar beet was studied under controlled conditions. The disease incidence increased with increasing tempera- ture. When the time up to emergence in a finesand soil was characterized by low temperatures (B°C night, 15°C day), a period of 14—21 days immediately after emergence caused a strong increase in disease incidence. In peat soil, even 7 days was enough to initiate such an increase. In neither of the soils used did three days of high post-emergence temperature cause any marked disease increase. When the pre-emergence period was characterized by high tempera- tures (15°C night, 25°C day) the disease was at a high level and in the very fine sand soil could not be lowered by lower temperatures. In the peat soil, however, a constant, low post- emergence temperature significantly inhibited the development of the disease. The possibili- ties of damping-off forecasting by comparing the disease incidence in pot experiments to damping-off in the field seemed rather limited. When the temperature is taken into account, a negative prognosis may be possible. The basis of such a prognosis is the determination of the inoculum potential of the soil. Introduction In Finland damping-off of sugar beet is mainly caused by the fungus Pythium de- baryanum auct. non Hesse (Vestberg et al. 1982). Of the factors affecting disease severi- ty and aggressiveness of Pythium spp. tem- perature is one of the most important. In vitro mycelial growth of Pythium debarya- num is optimal at about 25—28°C (Middle- ton 1943, Halpin et al. 1951). However, the fungus also grows well at very low tempera- tures, even down to +l°C (Middleton 1943). Under natural conditions, optimum soil temperatures for Pythium spp. are lower than those measured in vitro (Sverrison 1979, Lifshitz and Hancock 1983). The aim of this work is to study the effect of periods of high or low temperatures at and after emergence on the severity of damp- ing-off of sugar beet. Moreover, the possi- bilities are considered of forecasting disease severity by comparing disease percentages in pot trials with those in the field with the al- lowance made for the temperature factor. Index words: sugar beet, damping-off, temperature, disease forecasting 283 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=-BF7KvPEU8PUApnf.fwD0uEr7dMmY5U-_xP0bLA.2xvB0eB66-4SyNjy81b7mgeLCQqUhTudIBcvZDIQRYkNI09lBhX0vRU-hi3a8OTpB_d-R62DZTCHOBgTnj-SlRjMV6JDt47lCGI33FpvhsEOIV2rnT48Gtx213ju9D0qtIdmSqcZy5BaJQn1KcUv4bt02hS18dWphUD8oreW Materials and methods Temperature The effect of temperature on damping-off in sugar beet was studied using a naturally infested peat soil from Janakkala and a very fine sandy soil from Laitila. Two growth chambers were used, one with low (B°C night and 15°C day) and the other with high diur- nal temperatures (15°C night and 25°C day). During the experiment, pots were transferred from low to high temperatures and vice versa. The light intensity in the growth chambers was about 5500 lux and soil mois- ture was kept at about 70—90 % of water holding capacity. Disease forecasting In spring 1980, 36 sugar beet fields were chosen with varying degrees of damping-off according to earlier observations made by sugar beet advisors. Within each field, four areas of 3 x 5 m, i.e. 6 rows with a length of 5 m each, were chosen at random and marked so that exactly the same areas could be recovered the following year. In spring and autumn 1980 soil samples of about 10 1 each were collected from the four areas with- in each field. For practical reasons, the num- ber of fields on which damping-off could be reliably observed fell to 22 in 1981. Assaying for damping-off In the growth chamber experiment 0.5 1 plastic pots were sown each with 30 un- treated sugar beet seeds of the variety Mono- hill. After emergence readings were made on damping-off frequency at regular intervals until the end of the experiment. Seedlings with visible symptoms of damping-off were removed from the pots. Soil samples from fields chosen for disease forecasting were Table 1. The effect of pre- and post-emergence temperature on post-emergence damping-off of sugar beet. Growth chamber experiment. Low temperatures = +B°C night and + 15°C day. High temperatures = + 15°C night and +25°C day. Light intensity about 5500 Lux. Duration of experiment 35 days. Temperature Post-emergence damping-off % Up to After Very fine- Peat soil Mean emergence emergence sandy soil Low low 41 49 45.0 » high 94 96 95.0 » 3 days high 56 38 47.0 back to low » 7 days high 79 41 60.0 back to low » 14 days high 93 69 81.0 back to low » 21 days high 100 82 91.0 back to low High high 100 94 97.0 » low 85 39 62.0 » 3 days low 96 95 95.5 back to high » 7 days low 95 93 94.0 back to high » 14 days low 95 94 94.5 back to high » 21 days low 90 71 80.5 back to high 284 brought into the greenhouse for examination of their damping-off potential. After thor- ough mixing, 1 Vi 1 of each subsample from the field was put into two plastic pots and 35 untreated seeds of Monohill were sown in each. One of the pots was subsected to a low (B°C) temperature and the other to a high (18°C) temperature. After emergence, read- ings were made at about weekly intervals and diseased seedlings were removed from the pots. In the field, sugar beet advisors made damping-off observations about one week after emergence and again two weeks later. In the present experiments post-emergence damping-off was determined from the num- ber of diseased plants as a percentage of those emerged. No distinction was made between degrees of disease severity. Pre- emergence damping-off was determined as the difference in emergence between healthy control soil and diseased soil. Results and discussion Temperature The effect of temperature on post- emergence damping-off of sugar beet is pre- sented in Table 1. Seedlings grown continu- ously at low temperatures showed an infec- tion frequency averaging 45 %, while of those grown at consistently high tempera- tures, 97 % succumbed to post-emergence damping-off. When the temperatures up to emergence were low, a warm period of 14— 21 days duration starting immediately after emergence increased the disease incidence to about the same level as at constant high tem- peratures. A warm period of three days had no, or only a slight disease increasing effect. Seven days of high temperatures did not en- hance disease symptoms in the Janakkala soil, while in the very fine sand soil fromLai- tila this period of 7 days increased disease in- cidence significantly. With high temperatures from sowing to emergence followed by low temperatures till the end of the experiment, the percentage of damping-off dropped from 100 to 85 and from 94 to 39 in the Laitila and Janakkala soils respectively, as compared to cultivation at continuously high temperatures. Periods of 3—14 days at low temperatures had no disease decreasing effect whatsoever. In the Janakkala soil a cold period of 21 days had a slight disease decreasing effect (Table 1). The disease increasing effect of warm peri- ods is illustrated in Figure lA, where the Fig. I. Effect of low (A) and high (B) temperature pe- riods of varying lengths on the level of damping-off in a growth chamber experiment. Low temperature = + B°C night and +lB°C day High temperature = + 15°C night and +25°C day Light intensity about 5500 Lux The Janakkala soil is a peat soil and the Laitila soil a very finesandy soil. 285 length of the period is plotted against per- centage of post-emergence damping-off. The correlation coefficient is 0,938*** in the Ja- nakkala soil and o,Boo*** in the Laitila soil. In the opposite case (Figure IB) correlation coefficients for the two soils are —o,93l*** Table 2. Percentages of damping-off in 36 sugar beet fields in 1980 and in 22 fields in 1981 and oc- currence of post-emergence damping-off in the greenhouse in soil from the same fields. % damping-off In field 1980 36.8 (0.0—91.2) » » 1981 29.8 (2.7—80.4) In greenhouse at B°C Soil collected in spring 21.8 (3.4 —52.5) » » » autumn 31.3 (10.0 —64.8) In greenhouse at 18°C Soil collected in spring 32.5 (6.7—84.1) » » » autumn 61.6 (9.8 —98.3) and —o.964*** respectively when the length of the cold period is 0—36 days. The results indicate that temperature is a factor of considerable importance in deter- mining severity of damping-off under field conditions. The pathogenicity of P. de- baryanum in vitro is maximal at 25—28°C (Middleton 1943). Under natural conditions in the soil the optimum occurs at somewhat lower temperatures due to competition from antagonists at the higher temperatures (Sverrisson 1979). The results of this study are in accordance with the works by Buch- holtz (1938) and Nolle (1960). According to these workers, at soil temperatures above 15°C Pythium damping-offrapidly increases in aggressiveness. In Denmark, Mikkelsen (1982) made a survey of damping-off in sugar beet during 1954—1982. He found that in years with high temperatures at plant emergence and establishment the disease was 286 Table 3. Correlation coefficients between damping-off in the greenhouse of sugar beet cultivated in soil collected from 36 localities in 1980 and damping-off in the same fields in 1980 and 22 fields in 1981. Greenhouse Correlation coefficient field 1980 field 1981 Soil samples collected in spring Cultivation at B°C Post-emergence damping-off 0.066 0.274 Pre- + post-emergence damping-off 0.165 0.140 Cultivation at 18°C Post-emergence damping-off 0.292* 0.139 Pre- + post-emergence damping-off 0.182 —0.145 Soil samples collected in autumn Cultivation at B°C Post-emergence damping-off —0.109 0.439** Pre- + post-emergence damping-off —0.068 0.226 Cultivation at 18°C Post-emergence damping-off 0.424*** 0.287 Pre- + post-emergence damping-off 0.359** 0.357 4 Fig. 2. Correlations between damping-off in the field and in the greenhouse using soil collected from the same fields in spring and autumn 1980. Temperature in the greenhouse alternately +B°C and + 18°C. 287 288 more severe than in years with low tempera- tures. According to the present study the risk of serious outbreaks of damping-off is fairly small when temperatures after sowing are low. However, if emergence is followed by a warm period of 1 week or more the risk of serious outbreaks is considerable. On the other hand, if sowing is followed by a long period of very warm weather, there is hardly any chance of avoiding the disease. This ex- periment indicates that there are differencies between soils in their disease reaction to tem- perature variations. This is probably due to physical or biological properties of the soil, factors which need to be investigated further. Disease forecasting At the sites chosen for the disease fore- casting experiment damping-off averaged 36.8 °7o in 1980 and 29.8 % in 1981. Varia- tions between sites were considerable (Table 2). Soil collected from these fields in spring 1980 gave damping-off percentages of 21.8 and 32.5 in the greenhouse at B°C and 18°C respectively. Soil collected in autumn 1980 gave 31.3 and 61.6 % damping-off. Table 3 shows that in most cases the corre- lation for field soils between damping-off in the field and in the greenhouse is not signifi- cant, especially when the temperature in the greenhouse was held at B°C. On average there is a somewhat better correlation for field soil sampled in autumn than in spring. In 1980 there was a rather good correlation between damping-off percentages in the field and in the greenhouse at 18°C. In 1981, however, the best correlation was found for the low greenhouse temperature. The corre- lations were slightly higher when the disease incidence in the greenhouse was rated as post-emergence damping-off than as pre- + post-emergence damping-off (Table 3). In Figure 2 the percentage of field post- emergence damping-off in 1980 and 1981 is plotted against damping-off percentages in the greenhouse. As a rule, the points show a wide dispersion, and the feasibility of fore- casting damping-off in sugar beet thus appe- ars to be quite limited. This conclusion is in agreement with the work of Bartels and Winner (1971), who studied the Pythium infection of beet side roots with a view to making prognoses of damage by methods similar to those used in this study. The re- sults of the present investigation indicate some possibilities for a negative prognosis of the following type (Fig. 2D): Sugar beet plants should be grown under glass at a high temperature in soil collected from the field during the autumn. Fig. 4. Correlation between damping-off frequencies in soil samples collected in spring and autumn from sugar beet fields in 1980. Greenhouse temperature + 18°C. 1) If no disease occurs in the greenhouse, the risk of outbreaks occurring in the field is very small. 2) If heavy outbreaks occur in the green- house, the soil has a high inoculum poten- tial and there is a risk of severe attacks in the field. However, the disease may re- main at a low level if conditions unfa- vourable for development of the disease prevail. Apart from the infection potential of the soil, the severity of damping-off of sugar beet under field conditions depends on sev- eral other factors. The lack of correlation between 1980 and 1981 seasons for the level of damping-off in 22 beet fields shows that these other factors, in particular tempera- ture, are of great importance (Fig. 3). On the other hand, if conditions are standardized, infection potentials of soil samples can be compared to each other with fair reliability (Fig. 4). References Bartels, G. & Winner, C. 1971. Untersuchungen iiber den Einfluß wurzelinfizierender Pilze auf Entwick- lung, Ertrag und Qualität der Zuckerriibe unter dem Aspekt einer Schadensprognose. Zucker 24: 315—355. Buchholtz, W.F. 1938. Factors influencing the patho- genicity of Pylhium debaryanum on sugar beet seed- lings. Phytopath. 28: 448—475. Halpin, J.E., Hanson, E.W. & Dickson, J.G. 1951. Studies on the pathogenicity of seven species of Pyl- hium on red clover seedlings. Phytopath. 42: 245 249. Lifshitz, R. & Hancock, J.O. 1983. Saprophytic devel- opment of Pylhium ullimum in soil as a function of water matric potential and temperature. Phytopath. 73: 257—261. Middleton, J.T. 1943. The taxonomy, host range and geographic distribution of the genus Pylhium. Nem. Torrey Bot. Club 20: 1 171. Mikkelsen, N.U. 1982. Rodbrand og vaeltesyge i be- deroer. Botanisk Afdelning, Plantevaernscentret, Lyngby. Danmark. 33 pp. Nolle, H.-H. 1960. Über den Wurzelbrand der Zucker- riibe und seine Bekämpfung. Phytopath. Z. 37: 161— 200. Sverrisson, H. 1979. Pyt/imm-slacgtens biologi og plantepatologiske betydning. Den kgl. Veterinaer- og Landbohojskole, Plantepatologisk Afdelning. Dan- mark. 179 pp. Vestberg, M., Tahvonen, R., Raininko, K. & Nuorma- la, N. 1982. Damping-off of sugar beet in Finland. I Causal agents and some factors affecting the disease. J. Sci. Agr. Soc. Fin. 54: 225—244. Ms received December 4, 1984 289 SELOSTUS Sokerijuurikkaan taimipolte Suomessa. 111 Lämpötilan vaikutus tautiin ja sen ennuslamismahdollisuudet. Mauritz Vestberg Helsingin yliopiston kasvipatologian laitos, 00710 Helsinki 71 Vuodesta 1978 alkaen on Helsingin yliopiston kasvi- patologian laitoksen ja Sokerijuurikkaan Tutkimuskes- kuksen välisenä yhteistyönä tutkittu sokerijuurikkaan taimipoltetta. Tämä kirjoitus käsittelee lämpötilan vai- kutusta tautiin ja sen ennustamisen mahdollisuutta. Lämpötilan vaikutusta tutkittiin kasvatuskaapissa käyttäen luonnollisesti saastunutta turvemaata (Janak- kalasta) ja hienoa hietamaata (Laitilasta). Tutkimukses- sa selvitettiin taimelletuloa seuranneiden viileiden tai lämpimien ajanjaksojen merkitystä taimipoltteeseen, ottaen huomioon myös taimelletuloa edeltävä lämpöti- la. Matalassa lämpötilassa ( + B°C öisin, + 15°C päivi- sin) oli taimipoltetta 45 % ja korkeassa (+ 15°C öisin, + 25°C päivisin) 97 °/o. Taimipolte lisääntyi voimak- kaasti kun taimelletuloa edeltävän viileän ajanjakson jälkeen seurasi 14—21 päivän korkea lämpötila. Turve- maassa 7 päivän lämmin jakso taimettumisen jälkeen ei lisännyt tautia, mutta hietamaassa tauti lisääntyi voi- makkaasti tämän lämpimän jakson aikana. Sensijaan taimelletulon jälkeinen matala lämpötila alensi taimi- poltetta enemmän turvemaassa kuin hietamaassa. Vuonna 1980 valittiin ennustamiskoetta varten 36 so- kerijuurikaspeltoa eri puolilta Etelä- ja Lounais-Suo- mea. Näihin merkittiin jokaiseen 4 aluetta (3 x 5 m) si- ten, että alueet voitiin löytää myös seuraavina vuosina. Alueilta otettiin maanäytteet sekä keväällä että syksyllä 1980. Maanäytteissä kasvatettiin sokerijuurikkaan tai- mia sekä alhaisessa ( + B°C) että korkeassa (+lB°C) kasvihuonelämpötilassa. Tutkimus osoitti, että sokeri- juurikaspeltojen taimipoltteisuuden ja kasvihuoneessa kasvatettujen sokerijuurikkaiden taimipoltteisuuden vä- lillä oli heikko korrelaatio. Taimipoltteen ennustaminen on kuitenkin mahdollista seuraavalla tavalla: Jos multa- näytteissä kasvatettujen sokerijuurikkaissa ei esiinny taimipoltetta, on taudin ilmeneminen myös pellolla hy- vin pieni. Jos sensijaan multanäytteissä kasvatetuissa sokerijuurikkaissa on runsaasti taimipoltetta (korkea tautipotentiaali), on todennäköistä, että tautia esiintyy pellollakin runsaasti; Jos olosuhteet eivät ole kuiten- kaan suotuisat taudin kehittymiselle, vaikka maassa on runsaasti taimipoltetta, voi tautia olla vähän sokerijuu- rikkaissa. 290