Turf Fungicide Fundamentals  19
penetrants possess some qualities of both acropetal penetrants and contact fungicides. Residual surface deposits and accumulations in the waxy cuticle protect against new infections; translaminar movement of fungicide can suppress existing infections beneath the site of the fungicide deposit. Mobility of local and acropetal penetrants is illustrated in Figure 1.8, where four QoI fungicides were applied to the base of wheat leaves prior to inoculation with the powdery mildew pathogen. Mildew infection occurred above the fungicide application site for local penetrants (trifloxystrobin and kresoxim-Âmethyl), but infection all the way to the leaf tip was prevented where acropetal penetrants (azoxystrobin and picoxystrobin) were applied. Systemic penetrants. Systemic penetrants can move upward (acropetally) and downward (basipetally) in the plant. They are transported through the symplast, the continuous network of cells with living protoplasts. Phloem cells and mesophyll cells contain living protoplasts; epidermal and xylem cells do not. When a systemic penetrant fungicide diffuses through the cuticle and past epidermal cells, it migrates into mesophyll cells where it follows the transport of sugars into the phloem and other parts of the plant (Fig. 1.9). Sugars are essential for providing the energy and structural components for growth of all plant parts. The transport is passive and follows a gradient from areas of high sugar concentration to areas of low sugar concentration, or from source to sink (Fig. 1.10). Fully expanded leaves manufacture the most sugar. They require some of it to sustain their own growth, but they export most of it to other parts of the plant for storage or to be utilized for growth. Roots do not produce their own sugars, but they still must grow and rely on a regular
Fig. 1.8. Movement of acropetal and local penetrant fungicides. Fungicides were applied to the bases of wheat leaves prior to inoculation with the powdery mildew pathogen. (Adapted from Bartlett et al., 2002)