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Take-all root rot disease of wheat is caused by the soil-borne fungus Gaeumannomnyces graminis var. tritic. The fungus is present in many soils, but generally only becomes a problem under conditions of high rainfall or irrigation. In wheat the fungus attacks the roots, crown and basal stem tissue. Symptoms are most obvious during grain-fill and include stunting, reduced tillering, blackened roots and crowns (see photo below), premature ripening, and white heads with reduce kernel numbers and size. Root systems of severely infected plants frequently are so brittle that plants are easily pulled from the ground.

Soft white winter wheat studies in the Willamette Valley of Oregon were among the first to demonstrate the benefits of Cl fertilization to field grown crops (Christensen et al., 1981; Taylor et al., 1981; Christensen and Brett, 1985). Comparisons between different fertilizer N sources (e.g. (NH₄)₂SO₄; NH₄NO₃, urea, NH₄Cl) generally show that the highest production levels are achieved with NH₄Cl (Table 2).

Table 2. Soft white winter wheat yields as affected by fertilizer N source in western Oregon. Source: Christensen and Hart, 1993.

N Source¶	Growing season (number of experiments)

	1978(2)	1980(3)	1981(3)	1982(1)	1983(1)	1984(4)	1986(2)	1988(2)	1989(1)

	---------------------------------------------- bu/a # ------------------------------------------------
Urea						111a	93a	86a	144ab
NH₄NO₃						109b	94ab	93b	141ab
(NH₄)₂SO₄	54a	88a	66a	52a	106b	111a	99b	98b	138a
NH₄Cl	66b	107b	80b	70b	106b	111a	107c	116c	151b

¶ N was topdressed in the spring by Feekes growth stage 4 to 7. Rates were consistent within each experiment, but varied with growing season (ranged from 120 to 160 lbs N/a). # Within column means followed by the same letter are not significantly different (P=.05).

Christensen et al. (1990) concluded that enhanced host tolerance may be responsible for higher grain yields when take-all infected wheat is fertilized with Cl containing fertilizers. Recent studies indicated that NH₄Cl and (NH₄)₂SO₄fertilized wheat did not differ in take-all incidence and severity, or NH₄:NO₃ ratios in the soil, but did differ in yield. Further, application of dicyandiamide (DCD), an effective nitrification inhibitor increased the severity of take-all. Because the slope of the relationship between yield and take-all severity (Figure 1) is less negative for NH₄Cl than other N sources, it appears that Cl has increased that ability of the plant to tolerate severe take-all infections.