Part I. Noninfectious Diseases Biotic Agents of Noninfectious Diseases Turfgrasses are often damaged by the feeding activity of insects and by competition from weeds, moss, and algae. None of these pests is considered to cause true disease, but the disorders they cause may mimic disease. Insects that feed on crowns, roots, or stem bases or extract plant sap from foliage may cause plants to appear diseased. Careful diagnosis is required to assess the true cause of the damage. The task can be difficult if infectious agents become established in wounds created by insects or mechanical damage; in such cases, the primary cause of the disorder may rapidly become masked. Weeds, moss, and algae are usually quick to occupy voids that remain after turf has been thinned by infectious diseases or environmental stresses. Plants that invade weakened turf often mask the initial problem. Pesticides and management practices can control or reduce damage from most noninfectious pests.
algal growth and complement the recovery of grasses when wet conditions also are corrected.
Black-Layer
Blue-green algae (cyanobacteria) develop as a black scum on the surface of overly wet soils, particularly on golf and bowling greens (Figs. 3 and 4). The algae may reduce the exchange of gases between air and soil, which may induce chlorosis in plants and thinning of the stand. Algal scums can be controlled by improving soil and air drainage as well as irrigation procedures; maintaining proper soil pH and nutritional levels; increasing the mowing height; and increasing the penetration of sunlight through shrubs and trees. Some fungicides and other chemicals help to alleviate
Black-layer is a physical condition of soil, primarily associated with golf greens. Although it occurs in turfs grown on native soils, it is especially noted in turfs grown on substrates with high sand content. The major predisposing factor for black-layer is anaerobic conditions (lack of oxygen) in the soil. Algae may play a role in the development of some black layers by plugging pore spaces or sealing the surface. When soil becomes anaerobic, turf first develops a yellow or bronzed appearance and later thins in irregular patterns, especially in low-lying or shaded areas where air circulation is poor. Surface or subsurface black layers (Fig. 5) occur in the sand profile or in the thatch. Subsurface layers may develop 0.5–3.0 in. (1–8 cm) below the surface. Bands of blackened sand are usually 0.25–2.0 in. (0.6–5.0 cm) deep but have been observed as much as 7 in. (18 cm) deep. A foul, sulfurous, rotten egg odor is sometimes associated with affected soils. Black-layer is commonly associated with periods of excessive rain or irrigation, especially on golf and bowling greens with poor internal soil drainage or with a high water table. It often occurs where a sand topdressing is layered over a heavier soil or a thatch layer. Installing unwashed sod can also create soil layers that form a perched water table. Soils of low oxygen content are invariably associated with black-layer.
Fig. 3. Thin algal scum on a golf green. (Courtesy R. W. Smiley)
Fig. 4. Thick algal crust. (Courtesy M. C. Shurtleff, photo by M. P. Britton)
Algae
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