Symptoms and Signs
Symptoms of various virus diseases often resemble each other and may be difficult to differentiate in the field. Virus-infected leaves exhibit chlorosis, necrotic spots, mosaics, ringspots, mottling, crinkling (puckering), and other symptoms. Some viruses cause stunting and poor overall growth.
Symptoms of Cucumber mosaic virus infection on spinach include a slight chlorosis of younger leaves and narrow or "puckered" young leaves. Leaves also can have an inward rolling of margins. In advanced stages of disease, the plants often appear stunted and the crown leaves may become completely blighted, killing the growing point.
Beet western yellows virus infection appears as interveinal and leaf margin chlorosis on older leaves. As the disease progresses, chlorosis becomes more intense until the older, lower leaves become completely chlorotic. Older symptomatic leaves are frequently invaded by secondary fungi, resulting in necrosis that supports dark green to black fungal growth.
Beet curly top virus infections initially appear as leaf stunting and chlorosis. Younger leaves in the center of the rosette are often very chlorotic, extremely curled, and rigid. Plants usually die a few weeks after symptoms appear.
Impatiens necrotic spot virus and Tomato spotted wilt virus cause ringspots, circular leaf spots, and necrotic spotting to form on foliage. These diseases are not common in California spinach.
Tobacco rattle virus causes yellow and necrotic spotting, mottling, and leaf crinkling on spinach leaves.
Comments on the Disease
While there are over 14 reported virus diseases of spinach, only a few of these are economically important. Cucumber mosaic virus, Beet western yellows virus, and Beet curly top virus periodically cause minor to moderate damage in California. These three viruses have extensive host ranges that include many agronomic crops and weed species. Cucumber mosaic virus is vectored by aphids in a nonpersistent manner; Beet western yellows virus is also vectored by aphids but in a persistent or circulative manner and can persist in aphids for relatively long periods of time (4-6 weeks); and Beet curly top virus is vectored by the beet leafhopper in a persistent manner.
Other viral diseases are Impatiens necrotic spot virus and Tomato spotted wilt virus, which are vectored by thrips and are not common in spinach.
Tobacco rattle virus is vectored by soilborne nematodes and is also not common in spinach.
In inland areas, Cucumber mosaic virus can cause substantial damage in early fall-planted spinach. Throughout the summer in these regions, vector populations increase as the temperature rises, and many surrounding crops and weeds become infected with Cucumber mosaic virus. The chance of virus transmission to newly planted spinach during early fall is therefore high.
Weeds may act as virus reservoirs and should be removed. However, weed removal will not necessarily prevent virus infections from taking place. No resistant spinach cultivars are available. Spraying for the insect vector will not prevent virus infections from occurring; however, growers should still attempt to manage vector populations when possible (for more information, see section on APHIDS).