The effect of vesicuiar-arbuscular mycorrhizal inoculation on the growth and root colonization of ten strawberry cultivars Mauritz Vestberg Vestbero, M. 1992. The effect of vesicuiar-arbuscular mycorrhizal inoculation on the growth and root colonization of ten strawberry cultivars. Agric, Sci. Finl. 1:527-535. (Agric. Res. Centre ofFinland, Laukaa Res. and Elite Plant Unit, SF-41340 Laukaa, Finland.) Ten strawberry cultivars, four early maturing, three late maturing and three “special” cultivars, were inoculated with six strains ofvesicuiar-arbuscular mycorrhizal (VAM) fungi in a pot experiment. Growth effects and colonization of the VAM fungi were studied. Three strains. Glomus macrocarpum V3, G. mosseae Rothamsted and G. sp. V4, were highly efficient, causing significant growth increases in most cultivars. ’Jonsok’ showed the highest mycorrhizal dependency index, 648, and ’Ostara’ the lowest, 269, for the mean response ofall six fungi. The fungal strains which increased shoot growth the most also increased the runner plant formation the most. Early cultivars showed higher colonization percentages than late maturing cultivars. Sporula- tion of the introduced VAM fungi was on average more abundant in early and special cultivars than in late cultivars. Root colonization and strawberry shoot dry weight correlated significantly in most cultivars, but the correlation between colonization and runner formation was generally poor. Key words: VAM, strawberry cultivar, growth response, colonization, mycorrhizal dependency Introduction The variation in response of different plant species to vesicuiar-arbuscular mycorrhizal (VAM) infec- tion is well documented (Powell 1986). Dif- ferences in response to mycorrhizal infection is also observed among cultivars, for example in pea (ES- TAUN et al. 1987), wheat (Bertheau et al. 1980, Stöppler et al. 1990, Vierheilig and Ocampo 1991), peach (Traquair and Berch 1988) and apple rootstocks (Granger et al. 1983), maize (Hall 1978, Toth et al. 1990), cowpea (Rajapakse and Miller 1988), peanut (Daft 1991)and sorghum (Raju et al. 1990). Bryla and Koide (1990) studied therole ofmycorrhizal infec- tion in the growth and reproduction of wild ac- cessions and cultivars oftomato. In strawberry, the cultivar effect was studied by Robertson et al. (1988), who found first-year greenhouse plants of strawberry always to be non- mycorrhizal, but colonization of plants growing in fumigated foundation(2nd year) and certified fields (3rd year) to be highly variable between cultivars. However, in a glasshouse trial, all strawberry culti- 527 Agile. Sei. Fint. 1 (1992) https://www.c-info.fi/en/info/?token=A9faUqELR-CrX-9J.VtjG59OgyDJ6MDy1RE7jIA.U4HX20QHLgLz7Lz14qZffbSiz8DmVlfdXGpUCPEKF3PGZe0UTHw5EwFw2O_NjjRQr0_Na7x33adFKOxGOTQw1fb7fG-1x_NYU7RJ8SO75g3hs6SEhWwu2_cTJCkwBFQQASFpJ1lVCkpKMb16ww25of_xrhPId8Odg28dHHwwG2QYwoti6BvZTwQALWKV-nqj-yiddwoFsIpmqCPPPFWRr6OYYd7Wec1UuZINxhi7-JdYA9dBW7dM0kw54bTPT7LIv6CDxiqTMw vars were infected equally (Robertson et al. 1988). In Mexico, Chavez and Ferrera-Cerrato (1990) observed differences in strawberry cultivar responses to VAM fungal colonization. The objective of this study was to determine the effect of six VAM strains on ten strawberry culti- vars commercially grown inFinland. Differences in response between early maturing, late maturing and special cultivars were also evaluated. The experi- ment was carried out in a glasshouse. Material and methods Cultivars Ten cultivars were studied. Eight of these belonged to the species Fragaria x ananassa Duch. One cultivar was a F. Virginiana Duch. hybride (’Alaska Pioneer’) and one cultivar was a F. vesca L. (’Min- ja’). With respect to maturity they were divided into early, late and “special” cultivars. ’Jonsok’, ’Zefyr’, ’Kristina’ and ’Mari’ were early cultivars. The cul- tivars ’Bounty’, ’Hiku’ and ’Senga Sengana’ were regarded as late cultivars for Finnish growing con- ditions. The special cultivars studied were the ever- bearer ’Ostara’, as well as ’Alaska Pioneer’ and ’Minja’. Inoculum The inocula were produced by growing the VA-my- corrhizal fungi in maize (FI hybrid ’North Star’) roots for 103 days, from 24 April to 7 August, 1989, in a glasshouse. A steam sterilized (three times on successive days) sand was used as growth substrate for the inoculum. The substrate was limed to give a pH of approx 6.0, and fertilized with 2 g 1 1 bone meal (Daft and Nicolson 1966). Phosphorus-free Hewitt solution was applied once a week. The ino- cula, a mixture of infected maize root pieces with adhering sand, hyphae and spores were left to dry and stored at + 6°C until use. The strains and their origin, the number ofspores and the percentage of root infection were as follows; InoculumStrain VAM-% spores/gName Origin 1. Glomus mossae, Kent, UK Rothamsted 4 126 2. Glomus sp. V4, Kärsämäki, FIN ”thin-walled white” 48 1 3. G. macrocarpum Saarijärvi, FIN V 3 30 < 1 4. G. intraradices V2o Anjala, FIN 5. G. sp. V2l/88, Pelkosenniemi, "thin-walled white” FIN 27 < 1 49 9 < 1 386. G. mosseae VI lb Nilsiä, FIN Experiment The experiment was started in a glasshouse on 27 and 28 May, 1990, and it lasted 91 days. The growth substrate was a steam sterilized (1 hon three successive days) sand basically limed with 5 g f 1 dolomite lime to pH 6, and fertilized with 2 g 1 1 bone meal (Daft and Nicolson 1966). The sub- strate was stored open for at least two weeks after the steam sterilization. Micropropagated strawberry plantlets were planted in 2.5 dl plastic Vefi pots. The inoculum, (1 ml for each plant) which had been stored for 10 months, was placed into the planting hole prior to planting. After planting the pots were placed on top of an approx. 2 cm layer of wet vermiculite. During the experiment, the vermiculite substrate was watered and a phosphorus-free He- witt solution (0.1%) was applied. The experiment was of a split-plot design. During the experiment, the temperature in the glasshouse varied between +2O°C and +39°C. Observations Growth response at the end of the experiment was evaluated by the following criteria: - shoot dry weight including motherplant and run- ners - number of runners/mother plant - number of runner plants/mother plant 528 Agric. Sei. Fint. 1 (1992) Mycorrhizal dependency (MD) of strawberry cultivars was determined as the ratio of dry weight of inoculated plants to uninoculated plants multi- plied by 100. The percentage of infected root was determined by the gridline intersect method (GtO- vannetti & Mosse 1980). The abundance of spores in strawberry pots was estimated from a 50 ml root and soil sample. The sample was washed through a 0.5 and a 0.074 mm sieve and the amount of spores detected on the sieves was evaluated on a rating scale of 0-5 as follows: O = no spores 1 = very few spores (1-5/50 ml substrate) 2 = few spores (6-20/50 ml substrate) 3 = a moderate number of spores (20-several hun- dreds/50 ml substrate) 4 = spores abundantly 5 = spores very abundantly Results Shout growth Significant growthresponses due to VAM inocula- tion were observed in all cultivars (Fig. 1). Three strains, G. macrocarpum V 3 (3), G. mosseae (1) Rothamsted and G. sp. V 4 (2), were on average superior to the rest of the strains. G. macrocarpum V 3 (3) increased shoot dry weight the most in all cultivars. In ’Minja’, all isolates used caused a significant growth increase. In ’Senga Sengana’ and ’Mari’, only two out of six strains increased growth signifi- cantly. The rest of the cultivars lay between these extremes (Fig. 1). The groupof the three less efficient strains tended to stimulate the growth of special cultivars in par- ticular. G. intraradices V2O (4) increased growth significantly in all the special cultivars, in one of the late cultivars (’Hiku’) but in none of the early culti- vars. Glomus sp. V2l/88 (5) increased growth sig- nificantly only in the early cultivar ’Kristina’ and in the special cultivar ’Minja’, while G. mosseae (6) VI lb did so in the early cultivar ’Jonsok’, and in ali the special cultivars (Fig. 1). ’Jonsok’ had the highest mycorrhizal dependency index (MD), 648, and ’Ostara’ the lowest MD, 269, for the mean response of all six fungi. When MD was calculated for the most efficient strain, G. mac- rocarpum V 3 (3), it varied from 920 in ’Hiku’ to 366 in ’Mari’. When the least efficient VAM strain was used, the MD varied from 400 in ’Kristina’ to 138 in ’Hiku‘ and ’Ostara’ (Table 1). Table 1.Mycorrhizal dependency of ten strawberry cultivars. Mycorrhizal dependency (MD) Cultivar Most Least efficient efficient Average of strain strain 6 strains Early ’Jonsok’ 829 373 648 ’Zefyr’ 522 229 358 ’Kristina’ 614 400 470 ’Mari’ 366 196 281 Late ’Hiku’ 920 138 626 ’Senga Sengana’ 414 278 309 ’Bounty’ 618 200 471 Special ’Ostara’ 319 138 269 ’Alaska Pioneer’ 757 294 553 ’Minja’ 475 252 278 Runner formation With regard to runner plant production the straw- berry cultivars used in the experiment could be divided into three groups. The cultivars ’Hiku’, ’Mari’, ’Bounty’ and ’Kristina’ produced only a minimum amount of runner plants during the ex- perimental period. ’Senga Sengana’, ’Zefyr’, ’Jon- sok’, ’Ostara’ and ’Alaskan Pioneer’ produced ä moderate number of runner plants, while ’Minja’ produced runner plants abundantly. The results of ’Minja’ only are presented here. In ’Minja’, the three strains (G. macrocarpumV3 (3), G. mosseae Rothamsted (1) and G. sp. V 4 (2)) 529 Agric. Sei. Finl. 1 (1992) Fig. 1.The effect ofVAM inoculation on shoot dry weight and root colonization of ten strawberry cultivars. Pot experiment in the glasshouse. Early maturing, late maturing and ”special” cultivars are indicated by letters A, B and C, respectively. For inoculation treatments (0 = uninoculated, 1-6 = inoculated) see material and methods. Numbers inside bars indicate root colonization percentages. Within cultivars, bars of shoot dry weights marked with different letters differ significantly at P =0.05. 530 Sei. Finl. 1 (1992)Aghc. which caused the highest shoot growth increase, increased significantly also the numbers ofrunners and numbers of runner plants per motherplant (Fig. 2). However, also G. mosseae VI lb (6) increased significantly runner and runner plant production. Root colonization and speculation All cultivars tested showed mycorrhizal coloniza- tion. Only in two host-fungus combinations, i.e. for strain V2l/88 (5) in ’Kristina’ and ’Hiku’, no root colonization occurred (Fig. 1). G.sp. V 4 (2) caused the highest root infection, which varied from 42% in ’Hiku’ to 83 % in ’Senga Sengana’. The early cultivars had on average a higher colonization per- centage than the late cultivars (Fig. 1, Table 2). ’Mari’ and ’Jonsok’, both early cultivars, had on average (of6 VAM isolates) the highest root infec- tion levels, 42.44% and 42.09%, respectively. ’Hiku’, a late cultivar, had the lowest average VAM infection, 22.71%. Of the special cultivars, ’Alaska Pioneer’ had also a high average root infection which exceeded 40%. Due to a spread of VAM fungi, uninoculated plants were also infected in many instances. Out ofsix VAM strains, four sporulated under the prevailing conditions. The G. mosseae strain from Rothamsted (1) sporulated in all cultivars and G. macrocarpum (3) sporulated in all cultivars but ’Senga Sengana’ and ’Minja’. The highest average sporulation was found in ’Mari’ and ’Alaska pion- eer’. The average sporulation index was higher in the early maturing (0.88) and special (0.78) than in the late maturing (0.50) cultivars (Table 2). Correlations The strawberry shoot dry weights correlated signi- ficantly with VAM colonization in many cultivars (Table 3). A high correlation was found in the cul- Fig. 2. The effect ofVAM inoculation on the number ofrunners and the number ofrunner plants per mother plant in cultivar Minja. Pot experiment in the glasshouse. For inoculation treatments (0=uninoculated, 1-6 = inoculated), see material and methods. Bars of runners and number of runner plants, respectively, marked with different letters differ significantly at P =0.05. 531 Agric. Sei. Fin!. 1 (1992) Table 2. The effect ofVAM inoculation on root colonization and sporulation of ten strawberry cultivars as an average of six VAM isolates. Colonization percentages followed by different letters differ significantly at P = 0.05. Mean of6 VAM isolates Strawberry cultivar Root Sporulation colonization, % 0-5 Early ’Jonsok 42.09a 0.75 ’Zefyr’ 31.38ba 0.83 ’Kristina’ 22.97b 0.75 ’Mari’ 42.44“ 1.17 Late ’Hiku’ 22.71 b 0.42 ’Senga Sengana’ 34.86ba 0.50 ’Bounty’ 26.12b 0.58 Special ’Ostara’ 27.97b 0.58 ’Alaska Pioneer’ 41.13“ 1.17 ’Minja’ 30.90ba 0.58 Table 3. Significance ofcorrelation coefficients (r) of linear plots between percentage root colonization and shoot dry weight, the number of runners/mother plant and number of runner plants/mother plant in 10 strawberry cultivars. N = 35/cultivar. NS = not significant. *, **, and *** indicate significances at the 10%, 5% and 1% levels, respectively. Correlation between root colonization and Number Number of Shoot of runner dry runners/ plants/ Cultivar weight plant plant Early ’Jonsok’ *** NS ** ’Zefyr’ ** *** ** ’Kristina’ *** NS NS ’Mari’ * NS NS Late ’Hiku’ *** NS NS ’Senga Sengana’ NS NS NS ’Bounty’ * NS NS Special ’Ostara’ *** * * ’Alaska Pioneer’ NS NS NS ’Minja’ ** *** NS tivars ’Jonsok’, ’Hiku’, ’Ostara’ and ’Kristina’, a moderate correlation in the cultivars ’Zephyr’ and ’Minja’, a weak correlation in the cultivars ’Mari’ and ’Bounty’, and no correlation in ’Senga Senga- na’ and ’Alaskan Pioneer’. In contrast to this, run- ner formationand root colonization correlated only occasionally in a few cultivars. The numbers of runners per plant correlated significantly with VAM colonization in ’Zefyr’ and ’Minja’, and the number ofrunner plants per mother plant did so in ’Zefyr’ and ’Jonsok’. Discussion Differences in response of cultivars to VAM inocu- lation have been demonstrated in numerous studies. In extreme instances, cultivars within one plant species may response to VAM inoculation in vary- ing degrees, from highly increasing to decreasing response (Bertheau et al. 1980). Plant species which are normally mycorrhizal have even been made non-mycorrhizal by mutation (Due et al. 1989). However, lack of differences in cultivar re- sponse to VAM inoculation has also been reported (Granger et al. 1983, Traquair & Berch 1988). In the present study, all Finnish commercial strawberry cultivars responded positively to VAM inoculation during the prevailing experimental con- ditions in the glasshouse. However, there were dif- ferences in response to fungal strains, which stresses the importance of screening for mycorrhi- zal efficiency in VAM research. Ofsix strains, three were highly efficient, causing significant growth responses rather uniformly in all the cultiv-ars. However, a group of three other strains were less efficient, which resulted in differentpatterns of re- sponse in the cultivars. This group ofstrains tended to increase shoot growth of strawberry the most in the special cultivars, including the semi-wild ’A- laska Pioneer’ and ’Minja’ cultivars. This indicates a difference in response in wild or semi-wild vs cultivated genotypes of strawberry to VAM infec- tion. Such differences in response have been found also by Koide et al.(1988) and Bryla and Koide 532 Agric. Sei. Finl. 1 (1992) (1990) in oats and tomato. However, contrary to our study, they found that the cultivars were as a group more responsive to mycorrhizal infection than the wild accessions. Mycorrhizal dependency (MD) is used as an index to compare the receptivity of different plant species or cultivars to VAM fungi (Gerdemann 1975). The MD can vary from none to complete dependency. However, the MD of a host can be altered by a number offactors such as soil type, soil phosphorus content, mycorrhizal species, etc. (MENGEetaI. 1978,Azcon & Ocampo 1981).This was also demonstrated in the present study, where the MD index for strawberry cultivars varied con- siderably when calculated for the most or for the least efficient VAM strain. The MD ofthe cultivars varied from 648 in ’Jonsok’ to 281 in ’Mari’, but no distinct differences were observed between early, late or special cultivars when the MD was calcu- lated as an average for all six VAM strains. It can be concluded that all strawberry cultivars tested show a moderate mycorrhizal dependency under low- phosphorus conditions. A number ofstudies have been conducted to find out the reasons for differences in MD at the cultiv- ar level. Baylis (1975) reported that the length of root hairs and the thickness of roots can determine theMD level of a plant species or cultivar. Menge et al. (1978) suggested that the number of feeder roots in citrus cultivars might correlate with MD. In another study with citrus, Graham and Syvert- sen (1985) showed that the leaf P concentration, fineness ofroots, growth rate, hydraulic conductiv- ity, transpiration and COz assimilation rate were linked to MD. In cowpea, Rajapakse and Miller (1988) observed that the average length of fine roots was negatively correlated with MD. However, only 27 % of the variability in MD was explained by this variable. Therefore, they conclude that root morphology did not appear to determine the MD in cowpea. Early strawberry cultivars showed on average a higher root colonization and speculation than late cultivars. The reason for this remains unknown, because no root growth parameters were deter- mined in the present study. However, it is a well- known fact that cultivars differ from each other with respect to root morphology. Also root exuda- tion has been found to vary greatly between cultiv- ars. Azcon and Ocampo (1981) found that the absence of mycorrhizal infection in some wheat varieties was associated with lack of sugar exuda- tion from the roots rather than with the sugar con- tent of the roots. In their study, VA infection led to a decrease in thereducing and total sugar content of root extracts, and this effect correlated with the degree of mycorrhizal infection. The results of this study cannot be directly extra- polated to field conditions, where a much more complex system of microorganisms, indigenous VAM fungi, nutrients and soil animals prevails. However, in strawberry fields of low soluble P and low incidence of indigenous VAM fungi (such fields do occur), strawberry cultivars can be ex- pected to react to inoculationby differentarbuscu- lar mycorrhizal fungi in a similar manner to that of the present pot experiment. In conclusion, however, the cultivar response to mycorrhization under field conditions needs further study. References Azcon, R & Ocampo, J.A. 1981. Factors affecting the vesicu- lar-arbuscular infection and mycorrhizal dependency of thirteen wheat cultivars. N. Phytol. 87: 677-685. Baylis, G.T.S, 1975. The magnoloid mycorrhiza and myco- trophy in root systems derived from it. In; Sanders, F.E. et al. (eds.). Endomycorrhizas. Academic Press. London. Bertheau, Y, Gianinazzi-Pearson, V. & Gianinazzi, S. 1980. Developpement et expression I‘association endo- mycorhizienne chez le ele. I. Mise en evidence d‘un effet varietal. Ann. Amelior. PI. 30: 67-78. Bryla, D.R. & Koide, R.T. 1990. Regulation of reproduction in wild and cultivated Lycopersicon esculentum Mill, by 533 Agric. Sei. Finl. 1 (1992) vesicular-arbuscular mycorrhizal infection. Oecologia 84: 74-81. 1990. Role of mycorrhizal infection in the growth and reproduction ofwild vs. cultivated plants. 11. Eight wild successions and two cultivars of Lycopersicon esculen- tum Mill. Oecologia 84: 82-92. Chavez, MC.G. & Ferrera-Cerrato, R. 1990. Effect of Vesicular-Arbuscular Mycorrhizae on Tissue Culture- derived Plantlets of Strawberry. HortSci. 25: 903-905. Daft, M.J. 1991. Influences of genotypes, rock phosphate and plant densities on mycorrhizal development and the growth responses of five different crops. Agric. Ecosyst. Environm. 35: 151-169. & Nicolson, T.H. 1966. Effects ofEndogone mycorrhi- zae on plant growth. New Phytol. 65: 343-350. Due, G., Trouvelot, A., Gianinazzi-Pearson, V. & Giani- nazzi, S. 1989. First report of non-mycorrhizal plant mutants (Myc ) obtained in pea (Pisum sativum L.) and fababean (Vida faba L.). PI. Sci. 60: 215-222. Estaun, V., Calvet, C. & Hayman, D.S. 1987. Influence of plant genotype on mycorrhizal infection: Response of three pea cultivars. PI. Soil 103: 295-298. Gerdemann, J.W. 1975. Vesicular-arbuscular mycorrhizae. In: Torrey, J.G. & Clarkson, D.T. (eds.). The develop- ment and function of roots. Academic Press. London. 618 p. Giovannetti, M. & Mosse, B. 1980. An evaluation of tech- niques for measuring vesicular-arbuscular mycorrhizal infection in roots. New Phytol. 84: 489-500. Graham, J.H, & Syvertsen, J.P. 1985. Host determinants of mycorrhizal dependency of citrus rootstock seedlings. New Phytol. 101:667-676. Granger, R.L., Plenchette, C, & Fortin, J.A. 1983. Effect of a vesicular-arbuscular (VA) endomycorrhizal fungus (Glomus epigaeum) on the growth and leaf mineral con- tent of two apple clones propagated in vitro. Can, J. PI. Sci. 63:551-555. Hall, J.R. 1978. Effect of vesicular-arbuscular mycorrhizas on two varieties ofmaize and one of sweetcom. N.Z. J. Agric. Res. 21:517-519. Koide, R.T., LI, M.,Lewis, J. & Irby, C. 1988. Role of mycorrhizal infection in the growth and reproduction of wild vs. cultivated plants. I. Wild vs. cultivated oats. Oecologia 77: 537-543. Menge, J.A., Johnson, E.L.V. & Platt, R.G. 1978. Mycor- rhizal dependency ofseveral citrus cultivars under three nutrient regimes. New Phytol. 81: 553-559. Powell, C.L. 1986. Field inoculation with VA mycorrhizal fungi. In: Powell, C.L. & Bagyaraj, D.J. VA-mycorrhiza. CRC Press Inc. Boca Raton, Florida. 234 p. Rajapakse, S. & MillerJß. J.C. 1988. Relationship between cowpea root systems and mycorrhizal dependency. HortSci. 23: 568-570. Raju, P.S., Clark, R.8., Ellis, J.R., Duncan, R.R. & Ma- ranville, J.W. 1990. Benefit and cost analysis and phos- phorus efficiency ofVA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels. PI. Soil 124: 199-204. Robertson, W.J., Boyle, C.D. & Brown, H.L. 1988. Endo- mycorrhizal status ofCertified Strawberry Stock. J. Am. Soc. Hort. Sci. 113:525-529. Stöppler, H., Kölsch, E. & Vogtmann, H. 1990. Vesicular- arbuscular Mycorrhiza in Varieties ofWinter Wheat in a Low External Input System. Biol. Agric. & Hort. 7: 191-199. Toth, R., Toth, D., Starke, D. & Smith, D.R, 1990. Vesicu- lar-arbuscular mycorrhizal colonization in Zea mays af- fected by breeding forresistnee to fungal pathogens. Can. J. Bot. 68: 1039-1044. Traquair, J.A. & Berch, S.M. 1988. Colonization ofpeach rootstocks by indigenous vesicular-arbuscular mycorrhi- zal (VAM) fungi. Can. J. PI. Sci. 68: 893-898. Vierheiliq, H. & Ocampo, J.A. 1991. Susceptibility and ef- fectiveness ofvesicular-arbuscular mycorrhizae in wheat cultivars under different growing conditions. Biol. Fertil. Soils 1: 290-294. Manuscript received July 1992 Mauritz Vestberg Agricultural Research Centre ofFinland Laukaa Research and Elite Plant Unit SF-41340 Laukaa, Finland 534 Agric. Sei. Fin!. 1 (1992) SELOSTUS Vesikkeli-arbuskcli mykorritsasieniymppäyksen vaikutus kymmenen mansikkalajikkeen kasvuun ja mykorritsainfektioon MauritzVestberg Maatalouden tutkimuskeskus VA-mykorritsasieniymppäyksen vaikutusta kymmeneen Suomessa viljeltävään mansikkalajikkeeseen tutkittiin astia- kokeessa. Kaikkien lajikkeiden taimet olivat MTTK:n Lau- kaan tutkimus- ja valiotaimiasemalla tuotettuja mikrolisät- tyjä pikkutaimia. Kasvualustana käytettiin luujauholla (2 g l" 1 ) lannoitettua steriiliä hiekkaa. Taimille annettiin myös kerran viikossa fosforivapaata Hevvittin ravinneliuosta. Koe perustettiin 27.-28. toukokuuta 1990 ja se kesti 91 vuo- rokautta, Lajikkeet jaettiin aikaisiin (’Jonsok’, ’Zefyr’, ’K- ristina’ ja ’Mari’), myöhäisiin (’Bounty’, ’Hiku’ ja ’Senga Scngana’)ja erikoislajikkeisiin (’Ostara’, ’Alaskan pioneeri’ ja ’Minja’). Sienikantojen lukumäärä oli kuusi, viisi suoma- laista jayksi ulkomainen. Eri lajikkeille laskettiin myös my- korritsariippuvaisuusindeksi (MRI), joka saatiin jakamalla ympätyn taimen kuivapaino ymppäämättömän taimen kuiva- painolla jakertomalla tulos sadalla. Vallitsevissa olosuhteissa kaikki tutkitut mansikkalajik- keet hyötyivät VAM-ymppäyksestä, Kolme sienikantaa. Glomus macrocarpum Suomesta, G. mosseae Englannista ja määrittämätön Glomus-\a)\ V 4 Suomesta, olivathyvin tehok- kaita ja lisäsivät merkittävästi mansikalla sekä verson kasvua että rönsyjonojen ja rönsytaimien tuotantoa. Lajikkeiden välisiä eroja ei mainittavasti ollut. ’Jonsok’ oli keskimäärin eniten riippuvainen mykorritsaymppäyksestä (MRI =648) ja ’Ostara’ vähiten (MRI = 269). Ymppäyksen seurauksena aikaisilla lajikkeilla oli keskimäärin korkeampi juuriston prosentuaalinen raykorritsainfektio kuin myöhäisillä lajik- keilla. VAM-sienet muodostivat myös itiöitä runsaammin aikaisissa ja erikoislajikkeissa kuin myöhäisissä lajikkeissa. Useimmilla lajikkeilla juuriston mykorritsasienen prosen- tuaalinen runsaus korreloi sienen aiheuttaman verson kas- vunlisäyksen kanssa, mutta rönsyntuotannon kanssa oli mer- kitsevää korrelaatiota harvoissa tapauksissa. 535 Agric. Sei. Fin!. 1 (1992)