The effect of different levels of phosphorus and nitrogen on mycorrhizal growth response and infection was studied using Allium schoenoprasum and Glomus caledonium. Nitrogen was added as ammonium or nitrate salt. Both the level of soil phosphorus and the level of nitrogen added affected the mycorrhizal growth response, which was greatest at intermediate levels of P and N. The nitrogen source did not affect the mycorrhizal growth response. At low levels of soil P, nitrogen addition did not affect mycorrhizal infection rate. High P and low N also had little influence. However, the combination of high P and high N gave much lower levels of mycorrhizal infection compared with the other treatments. This effect was most pronounced with ammonium N compared with nitrate N.

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The effects of eight plant protection chemicals on the development of vesiculararbuscular mycorrhizas were influenced by the identity of the host plant and fungal endophyte, the soil type, and the composition and concentration of the biocide. These effects ranged from stimulatory (as with the fungicide chloroneb, applied to inocula of Glomus microcarpus used with pot-grown lettuce), through negligible (aldicarb on G. mosseae in pot-grown onions and field grown barley with indigenous mycorrhizal fungi) to short-term inhibition (drazoxolon on G. mosseae in onion) and longer term inhibition (the fungicides benomyl and triadimefon on G. mosseae in onion).

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