Reports on Plant Diseases

COMMON, OR PSEUDOPEZIZA LEAF SPOT. This is caused by the fungus Pseudopeziza medicaginis and occurs wherever alfalfa is grown. The disease appears to be most serious on soils that are acid or low in fertility. Plants may be severely weakened, lack vigor, and become stunted the first year, but little permanent damage occurs. Common leaf spot starts on the lowermost foliage and progresses up the plant. Later cuttings are usually the ones attacked most severely.

Small, circular, dark brown to black spots, about 1/16 inch (1 to 3 mm) in diameter, develop first on the lower and inner leaves (Figure 1). In the thickened center of fully developed spots, a tiny, raised, light brown, disk-shaped fungus fruiting body (apothecium) forms on the upper leaf surface. This fruiting body is easily visible with a hand lens or reading glass. Large numbers of microscopic spores (ascospores) are shot into the air during cool to warm, wet weather and are carried by wind currents and rainsplash to other plants. Thus, the disease may spread quickly throughout a field. When the spots are numerous, the leaflets soon turn yellow and fall off. Premature defoliation reduces the quality and quantity of forage. The causal fungus overwinters as mycelium in fallen, undecayed leaves and leaf fragments on the soil surface. It is not known to be seedborne. As indicated, disease attacks are most damaging on susceptible varieties during prolonged periods of cool, wet weather in spring, early summer, and autumn. The incidence of disease decreases during dry warm summers.

Figure 1. Common or Pseudopeziza leaf spot.

SPRING BLACK STEM AND LEAF SPOT. All aboveground parts of the plant are attacked by the fungus Phoma medicaginis var. medicaginis (formerly called Ascochyta imperfecta and Phoma herbarum var. medicaginis). Numerous spots develop on the lower leaves, petioles, and stems in early spring. These spots are small, dark brown to black, and irregular. Young shoots are often girdled and killed. Leaf lesions may enlarge and merge, killing large areas of the leaflets. The leaves turn yellow and often wither before dropping off. Stem and petiole lesions may enlarge, girdle, and blacken large areas near the base of the plant (Figure 2). The fungus may extend into the crown and upper root causing a crown and root rot. Affected stems are brittle and easily broken. When severe, entire stems are blackened and killed.

The Spring Black Stem and Leaf Spot fungus (Phoma) mostly overwinters as mycelium in old stems and fallen leaves where minute, brown to black, pimple-like fruiting structures (pycnidia) are produced. Large numbers of microscopic spores (conidia) formed within the pycnidia, ooze out in cool, wet weather and are spread primarily by splashing water, but also by wind and insects. In cool humid areas, seed pods may become infected and the fungus becomes seedborne as mycelium in the seed coat. Infection of new alfalfa shoots by the Spring Black Stem and Leaf Spot fungus occurs as the shoots grow through the residue or stubble of a previous alfalfa crop. The first harvest is usually damaged the most.

Figure 2. .Spring black stem.

YELLOW LEAF BLOTCH is caused by the fungus Leptotrochila medicaginis (formerly named Pseudopeziza or Pyrenopeziza medicaginis); imperfect stage Sporonema phacidiodes. It occurs primarily in the northern half of the United States and in Canada. Yellow leaf blotch is less prevalent than common leaf spot or spring and summer black stem, being most conspicuous in rank and unusually tall stands. Disease attacks occur during prolonged cool, wet weather in the spring and autumn. Enlarging yellow spots, streaks, and blotches develop, most commonly along the leaf margins or along the veins. The blotches become elongated to fan-shaped as they expand and orange-yellow with small dark dots, which are fungus fruiting bodies (pycnidia), embedded in the center. The center of the blotched area finally becomes dark brown to black with the formation of pseudostromata and apothecia (Figure 3). Similar, elongated yellow blotches may occur on the stems. These blotches later turn dark brown. Dead leaves curl downward and frequently remain attached to the stems for some time.

The yellow leaf blotch fungus overwinters as mycelium and apothecia in infected leaflets and stems. It is spread mostly by planting infected seed and by airborne ascospores. The discharge of ascospores is favored by a relative humidity above 70 percent and a temperature below 77 F (25 C). The ascospores germinate from 36 to 88 F (2 to 31 C) with an optimum of 40 to 70 F (7.5 to 21 C). Their germ tubes penetrate alfalfa tissue directly within 4 hours at 68 F (20 C), 8 hours at 54 F (12 C), and 24 hours at 43 F (6 C).

Yellow leaf blotch attacks are favored by prolonged cool, wet spring weather, a thick nurse crop, and succulent, lush growth. Losses are in proportion to the number of leaves that drop during wind and rain storms, which may be as high as 40 percent at harvest time or 80 percent at the time of seed pod formation.

Figure 3. Yellow leaf blotch. Note the dark areas (pseudostromata and apothecia) on the affected leaflet (courtesy of Clemson University and USDA).

LEPTO, OR PEPPER LEAF SPOT is caused by the fungus Leptosphaerulina (Pseudoplea or Pleosphaerulina) briosiana. Lepto leaf spot is common throughout the Midwest, especially during prolonged periods of cool, moist weather. Small, irregular black spots develop–mostly on the younger leaves, although some may form on the petioles and young stems. Leaf symptoms vary with the plant's age, stage of growth, and environment. The lesions often start as small, black "pepper" spots. The lesions generally enlarge to form round to oval, light brown to tan spots with a darker brown margin–often surrounded by a yellowish area (Figure 4). Under prolonged cool, wet conditions with rapid regrowth, the lesions appear as rather large, light tan to almost white areas that merge to kill the entire leaf. In older growth, the young upper leaves become infected and have typical symptoms but seldom die before harvesting. The dead leaflets and petioles often cling on the stems for a time. Tiny black dots (fungus fruiting bodies called ascocarps) may be sprinkled within the light brown centers in the late fall and early spring. Ascospores are produced within the fruiting bodies on dead but undecayed leaves and stems during cool, moist weather. The ascospores are forcibly discharged and disseminated by air currents. The greatest damage by Lepto leaf spot occurs to the leaves of young regrowth following harvest. Epidemics, however, may occur at any time during the growing season following periods of cool, moist weather.

Figure 4. Lepto, or pepper leaf spot.