Pasmo (fungus – Septoria linicola): This is probably the most important disease of flax in Texas. Lesions develop first on the cotyledons and later on the lower leaves of the seedling. Lesions are usually circular and vary in color from greenish yellow to dark brown, depending upon age. Later, stem lesions develop, first as small elongated lesions which then enlarge and coalesce, extending around the stem as well as longitudinally. The infected areas alternate with green tissue until infection becomes severe; then the stems brown as the plants are defoliated. There is no evidence that the fungus oversummers in Texas, so it is likely that the disease is primarily seed-borne. Rotation and removal of straw or covering the straw with plowing helps reduce the inoculum. Use seed treatment and resistant varieties.
Rust (fungus – Melampsora lini): This disease can be readily recognized by the reddish, raised spots on leaves and stems. Early in the season, the rounded bright orange pustules can be seen on leaves. Later, the pustules turn black, causing defoliation of plants. The disease is favored by cool, moist weather. It is spread by spores produced in the pustules. The rust organism overwinters in flax straw, from which spores are produced that cause primary infection. Susceptible varieties such as De Oro should not be grown in Texas. New races of the fungus develop; therefore, epidemics are likely to occur until new resistant varieties are developed.
Aster Yellows(mycoplasma): The causal agent is transmitted from diseased to healthy plants by the six-spotted leafhopper. Infected plants are bright yellow with considerable distortion of foliage and floral parts. Flower parts turn green, forming a rosette-like growth instead of a boll. Seeds are shriveled or do not form. There is no effective control for this disease. Dense plantings tend to reduce the incidence of disease, since the insect vector prefers plants that are not crowded.
Curly Top(virus): The vector of this disease is the beet leafhopper, an insect which prefers arid conditions; therefore, the disease is more prevalent during dry years. Infected plants show a characteristic erectness of leaves about the stem. Plants may be yellow or reddish in color and the lower leaves drop prematurely. Later, plants branch abnormally, sometimes with tops twisted. Plants affected by curly top are more susceptible to Rhizoctonia root rot. There is no effective control for this disease.
Wilt (fungus – Fusarium oxysporum f. sp. lini): The fungus can attack flax plants at any stage of growth. In the seedling stage, roots are ashen-gray in color and the small plants wilt and die. In older plants, leaves turn yellow at the tip first, turning completely yellow later on. The disease is seed and soil-borne, and affects plants mostly when temperatures are high. Since flax in Texas is grown in the winter, the disease has not caused serious problems. Most common varieties are resistant.
Boll Blight(fungi – various organisms): Several fungi are involved in the complex disease. The fungi invade the capsule, turning the pod black and causing discoloration and rotting of grain. The disease is enhanced by insect damage and humid, wet conditions. Some varieties are more resistant to the disease because of the structural shape of the pod, which deters the entrance of organisms.
Other disorders: Mineral deficiencies, particularly iron and zinc, occur commonly in Texas. Zinc deficient plants show death of the terminal bud, profuse tillering and necrosis of the leaves. Plants are small and unthrifty with reduced yields.
Another condition, late season decline, results in the wilting of plants brought about by drought stress during the flowering stage.