Identification and physiological significance of mycorrhizal associations of apple (Malus domestica Borkh.)

Vesicular-arbuscular mycorrhizae of apple (Malus domestica Borkh.) were surveyed. Soil and root samples were collected in August 1980 from 18 planting sites of the U.S.D.A. North Central Region Cooperative Project 140 apple rootstock trial. Species of endogonaceous fungi were identified from soil samples, with a range of 3 to 8 among sites and no apparent patterns of geographic distribution. Glomus was more common in number of species and number of spores than Gigaspora, Acaulospora or Sclerocystis. Root samples were endomycorrhizal at all sites. Root colonization percentage among sites was negatively correlated with soil zinc and (at most sites) phosphorus and was correlated with soil infective potential as determined by a dilution study. Spore counts were a poor indicator of soil infective potential with apple;Six species of endogonaceous fungi originally isolated from survey soils were evaluated at low P (30 mg/kg) and high P (180 mg/kg) for root colonization and effects on growth of apple seedlings in the greenhouse. Fungal species evaluated were Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, Glomus maculosum sp. ined. Miller and Walker, Glomus manihotis sp. ined. Howeler, Sievarding, and Schenck, Gigaspora calospora (Nicol. and Gerd.) Gerdemann and Trappe, Glomus bitunicatum sp. ined. Walker and Miller, and two isolates of Glomus occultum Walker. Trees grew approximately two times larger and had higher leaf P concentrations at high P than at low P regardless of mycorrhizae. At low P, three fungal species, Glomus mosseae, Glomus manihotis, and Glomus occultum (Indiana isolate) were superior symbionts in stimulating apple tree growth as compared to the non-mycorrhizal controls. Fungal species varied in root colonization characteristics and spore production. Root colonization percentage was well-correlated with final height and less correlated with total dry weight. At high P, no treatment resulted in greater than 5% of root length colonized and endogonaceous fungi had little apparent influence on plant growth