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Symptoms and Signs

Symptoms

CMV infects 1200 species in over 100 plant families and can cause significant economic losses in many vegetable and horticultural crops. CMV causes a systemic infection in most host plants, but may remain symptomless in some crops like alfalfa. Symptoms of cucumber mosaic can vary greatly depending on the crop infected and the age of the plant when infection occurs.

Cucurbits: Almost all cucurbits are susceptible to CMV, with symptoms varying in severity (Figure 3A-E). Severe epinasty, downward bending of the petiole and leaf surface along with leaf reduction, are common in early season infection of summer squash (Figure 3A).

Figure 3 A-E. CMV infection of cucurbits

Plants infected early in the season are severely stunted and leaves are malformed, and fruit are unmarketable because of pronounced rugosity (roughness) on the fruit surface, as shown on the infected zucchini plant and fruit in Figure 3B. Infection of vining crops, such as muskmelon, show severely stunted growing tips (Figure 3C), and although fruit may not show symptoms they are of poor quality. If the yellow squash variety grown lacks the precocious gene, color breaking will occur on the fruit, causing the fruit to show green blotchy patterns, but these symptoms are absent in yellow squash varieties with the precocious gene (Figure 3D). Color breaking on fruit of varieties without the precocious gene will also occur with Watermelon mosaic potyvirus (WMV) infection; however this protection does not hold true for Papaya ringspot potyvirus or Zucchini yellow mosaic potyvirus, where both foliage and fruit of yellow squash are severely affected. Pumpkin is another cucurbit, that when infected at any early stage, will express severe foliar mosaic and the fruit will show a mosaic pattern and would be unmarketable (Figure 3E).

Figure 4 A-I. CMV symptoms on non-cucurbit crops

Pepper: Foliar symptoms of pepper plants vary with stage of infection. The initial flush of systemic symptoms typically includes a chlorosis of young leaves that may occur over the basal portion of the leaf or over the entire leaf. Oak leaf and ringspot patterns may develop on these leaves as the plant ages (Figure 4A). As new leaves emerge, these leaves develop a chlorotic mosaic pattern that tends to encompass the entire leaf. Leaves that develop subsequent to those expressing the chlorosis and chlorotic mosaic symptoms may have varied degrees of deformation including sunken interveinal lamina with protruding primary veins. These leaves also have a dull light green appearance as opposed to the dark green, rather shiny leaves of healthy pepper plants (Figure 4B). These symptom patterns vary in severity depending on the age of the plant at the time of infection with more severe symptoms typically resulting when plants are young at the time of infection. CMV infected plants also tend to be stunted with those plants infected early in development possibly expressing severe stunting, whereas plants infected at later stages of development may have little, if any, stunting. Pepper fruit may develop ringspotting and roughness leading to unmarketable fruit (Figure 4C).

Spinach: CMV infection of spinach is often referred to as spinach blight. The symptoms can vary depending upon the variety, plant age when infected, temperature, and virus strain. Typical symptoms include leaf chlorosis (Figure 4D), which can progress to cause severe blighting of the growing point and eventual plant death. In addition to chlorotic mottle, leaves can show narrowing, wrinkling with vein distortion, and inward leaf roll.

Lettuce: Symptoms of CMV infection of lettuce consist of leaf mottling, severe roughness of the leaf and occasional necrosis within the leaf tissue (Figure 4E). Plants are usually stunted if infected at an early stage of development.

Celery: CMV infection of celery is referred to as southern mosaic virus in the older literature following its description in southern production areas. Initially leaves will develop veinclearing and mosaic, and later these same leaves may show yellowing and veinal necrosis (Figure 4F). The petioles of these plants show elongated, brown to translucent, sunken beige colored lesions (Figure 4G), making the celery stalk unmarketable. Symptoms may also be transient, such that later in the season the symptoms may be limited to a dull cast, while under cool growing conditions the obvious symptoms may be muted.

Tomato: Tomato plants infected with CMV in the early stages are yellow, bushy and considerably stunted. The leaves may show a mottle similar to that caused by Tomato mosaic virus (ToMV). The most characteristic symptom of CMV is filiformity or shoestring-like leaf blades (Figures 4H and 5).

Figure 5. “Shoestringing" of CMV infected tomato

This could be confused with a symptom of ToMV known as "fern-leaf" in which the blade of the leaflet is not as completely suppressed as in a "shoestring" leaflet, but is abnormally long and narrow. The symptoms caused by CMV can be transitory, with the bottom leaves or newly developed top leaves showing severe symptoms, while the middle leaves may appear almost normal. Severely affected plants produce few fruit, which are usually small, often mottled or necrotic, with delayed maturity.

CMV symptoms in tomato have two distinct forms depending on growing conditions.
In the greenhouse. CMV-infected tomato plants grown under greenhouse conditions may express a general chlorosis followed by an easily observed mosaic symptom on young, systemically infected leaves. These symptoms tend to be somewhat transient as newly emerging leaves may be smaller than normal and show varying degrees of deformation such as shoestringing. Internode elongation is reduced leading to overall stunting but a bushy growth habit may not occur.
In the field. The occurrence of mosaic symptoms on young, systemically-infected leaves typically include initial chlorosis and mosaic. If plants are young at the time of infection, the mosaic symptoms are rather transient due to severe leaf deformation, and in particular, an upward cupping or rolling of leaves. If plants are somewhat more mature at the time of infection, mosaic symptoms may be more obvious, in part, because such leaves are not masked by severe leaf deformation and cupped or rolled leaves. Tomato plants that are young at the time of infection remain small, sometimes severely stunted, with a bushy appearance.

Bean: Symptoms of CMV infection in bean consist of leaf curl, green mottle and blistering, and a zipperlike roughness along the main veins involving only a few leaves (Figure 4I). Foliar symptoms are most obvious and pod infection and loss is greatest when plants are infected before bloom. Early infected plants may yield no or few pods because CMV caused flower abortion and abnormal development. The pods are mostly curved, mottled and reduced in size. Plants may recover and resume normal growth with limited yield loss if plants are infected after bloom. Because symptoms can vary among varieties and can be confused with those caused by Bean common mosaic virus, it is best not to rely upon symptoms to diagnosis the particular virus(es) involved. Rather the serological technique called ELISA (enzyme-linked immunosorbent assay) should be used to confirm if the plant is infected with CMV or another virus.

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