Influence of Ammonium Sulfate and Rotation Crops on Strawberry Black Root Rot

The combined effects of rotation crops and nitrogen fertilizers were examined on the strawberry black root rot disease complex. In July 1995, microplots were filled with soil that had a history of strawberry black root rot and seeded with two types of oats (Avena strigosa 'Saia oats' or A. sativa 'Garry oats') or with sorgho-sudangrass (Sorghum bicolor × S. sudanense 'Triple S'). Microplots planted with 1-year-old 'Honeoye' strawberry crowns served as the controls. In May 1996, the crops were chopped and incorporated into the soil. The soil was re-planted with 1-year-old strawberry 'Honeoye' crowns and then fertilized with (NH₄)₂SO₄ or Ca(NO₃)₂ at equivalent rates of N. Two months later, (NH₄)₂SO₄-treated plants had 36% more leaf area and 41% more runners than strawberries treated with Ca(NO₃)₂ Strawberries that had been pre- cropped with 'Saia' oats had 135% more runners and 38% more early fruit yield than strawberries grown in control microplots. Total fruit yield was not affected by the treatments. Compared to Ca(NO₃)₂, the (NH₄)₂SO₄ treatment reduced the percentage of blackened roots. The influence of the cover crops on growth and disease was stronger with (NH₄)₂SO₄ fertilization than with Ca(NO₃)₂ fertilization. Combining 'Saia' oats or sorgho-sudangrass rotation with (NH₄)₂SO₄ fertilization reduced lesion nematode (Pratylenchus penetrans) numbers in subsequent strawberry roots when compared to controls. Also, the combination of 'Saia' or 'Garry' oats as a pre-crop with applications of (NH₄)₂SO₄ reduced black root rot severity when compared to controls. Only the 'Garry' oat rotation reduced strawberry root colonization by Rhizoctonia fragariae when compared to controls. Other effects were associated with using (NH₄)₂SO₄. The (NH₄)₂SO₄ treatment lowered the rhizosphere soil pH by 0.2 units, reduced the numbers of fluorescent pseudomonads in the rhizosphere by 10- to 15-fold, and produced leaves that had more N, K, S, Mn, and Zn content than plants treated with Ca(NO₃)₂. Rotation with 'Saia' oats combined with (NH₄)₂SO₄ fertilization may suppress strawberry black root rot and increase yields through multiple effects on the host, pathogens, and associated microflora.