John F. Murphy, Edward Sikora, Steven Slack, Michael Guerini, Lewis Tapley, and Edward Tunnell

Irish potatoes represent the largest acreage of any vegetable crop grown in Alabama, and Alabama is the leading producer of summer-season potatoes in the southeastern United States. Research conducted through the AAES is helping ensure a healthy crop by identifying viruses that can potentially devastate this important crop.

As part of an integrated pest management (IPM) program in Irish potato, a survey was conducted to determine the occurrence of plant viruses that commonly affect potato production in the United States. The survey included commercial production fields in Baldwin (South Alabama) and DeKalb and Jackson (North Alabama) counties. AAES researchers identified six different viruses from potato foliar tissue samples. Four of the viruses occurred as a mixed infection involving two or more viruses within a single plant.

Commercial Irish potato production is located primarily in Baldwin County in the southwestern portion of the state and in Cullman, DeKalb, and Jackson counties in the northern portion of the state.

The AAES survey was conducted over a two-year period to determine whether viruses commonly identified in other potato growing regions in the United States also occurred in Alabama. The goal of the research was to determine which viruses, if any, occurred in commercial potato plantings. Armed with this information, scientists can develop management strategies and preventative measures to avoid extreme outbreaks of the most commonly occurring viruses.

In 1997, potato fields were surveyed in Baldwin (in May) and in DeKalb and Jackson (both in June) counties corresponding to a period just prior to vine kill (approximately 85-100 days after planting).

Twenty potato plants in each field were evaluated visually for the occurrence of virus-like symptoms. A leaf sample, consisting of three leaves per plant collected randomly over the upper portion of the plant canopy, was collected from each of the 20 plants and the leaf samples were tested for each of eight viruses by the serological test enzyme-linked immunosorbent assay (ELISA). The eight viruses included cucumber mosaic cucumovirus (CMV), potato potyviruses A (PVA) and Y (PVY), potato carlaviruses M (PVM) and S (PVS), potato potexvirus X (PVX), potato leafroll luteovirus (PLRV), and tobacco etch potyvirus (TEV).

Six viruses were detected from potato leaf samples (see the table): CMV, PVA, PVS, PVX, PVY, and PLRV. In South Alabama (Baldwin County), virus-infected potato samples occurred in six of seven fields. Four different viruses were detected: PLRV, PVS, PVX, and PVY. PVY was detected in five of the seven fields surveyed and occurred most frequently among samples from those fields. PLRV was detected in samples from three fields; PVS and PVX each were detected in samples from two fields. Mixed infections occurred in samples from three of the seven fields (see table). Potato plants infected with the viral combinations PVX+PVY or PVS+PVY+PLRV, were severely stunted with red, blistered leaves; however, one plant infected with PVX+PVY displayed no symptoms.

In DeKalb and Jackson counties, virus was detected in five of the seven fields surveyed (see the table). Five different viruses were identified: CMV, PVA, PVS, PVX, and PVY. PVS occurred most frequently among samples and in the number of fields surveyed (detected in four of seven fields). CMV, PVA, PVX, and PVY were each detected in potato samples from only a single field. Mixed infection occurred in only two fields and included PVS+PVX and PVS+PVY. No virus was detected from any of the tested samples from two of the fields in DeKalb County.

The study did identify viruses in commercial plantings of Irish potato; however, researchers were less successful at correlating virus-like symptoms in the field with identification of a specific viral causal agent. Generally, PLRV-infected plants always had red, upward curled leaves, and represented the only virus disease that could be identified with confidence (see the photos). In contrast, plants with chlorotic (faded, paler green) and/or mosaic leaves tended to be infected with either PVX, PVS, PVY, or combinations of the three viruses. CMV was detected in only two plants and each was symptomless. Similarly, some of the plants infected with PVA, PVS, or PVX, based on detection by ELISA, were symptomless, which apparently is not unusual for PVS and PVX infections in potato cultivars grown in North America.

The upward curled leaves seen in these photos are characteristic of the potato leafroll disease caused by potato leafroll lutoevirus. The diseased plants illustrate different degrees of potato leafroll disease severity, the photo on the right being most severe.

The difficulties associated with identification of a specific virus infection based on appearance of some characteristic symptom(s) were compounded by the occurrence of mixed infections. For instance, some potato plants were clearly infected with PLRV, based on appearance of characteristic symptoms; however, ELISA tests revealed the presence of one or two other viruses in the same sample whose symptoms were masked by the PLRV infection.

A recurring epidemic of CMV (and to a lesser extent TEV) in fresh-market tomatoes occurred during the months of June and July in Blount and St. Clair counties, which border potato production areas. Because potato is an early season crop and reaches maturity in this region sometime during June, with harvest occurring in July, it seemed possible that potatoes might play a role in the spread of these viruses to tomatoes. However, results of this study indicated that very little CMV was detected in potato. Thus, potato is not likely to serve as a source of CMV inoculum for crops grown later in the season.

Though an accurate determination of viral disease incidence in potato plants may not be possible by simply looking at a field or at individual plants, this study established for the first time that viruses commonly identified in other potato growing areas of the United States also occur in Alabama. This information represents an essential starting point for development of management strategies.