What Is It? | Facts in Depth | For the Professional Diagnostician
Symptoms begin in individual leaves, where they suddenly wilt and eventually shrivel (Smith, 1911). Early in the disease, leaves may “droop” during the heat of day but revert to normal in the evening or under cooler conditions. Leaves show a darker green color around the infected area and a few leaves show wilt along the stem. As wilting progresses, the xylem vessels inside the stem collapse and are discolored. Eventually, the entire plant turns to a dull green color, collapses, and eventually dies (Smith, 1911; Koike et al., 2007).
(Left: Pumpkin; Middle: Zucchini; Right: Melon)
Erwinia tracheiphila survives in the digestive system of the striped and spotted cucumber beetle vectors (Acalymma vittatum (F) and Diabrotica undecimpunctata howardi Barber, respectively). The pathogen does not overwinter in volunteer crops or weeds such as curley dock, goldenrod, giant goldenrod, johnson grass, buckhorn plantain, and shepherd’s purse. The cucumber beetles remain dormant in the soil until springtime, when they reemerge and become a threat to cucurbit fields. The pathogen is transmitted through feces that enter wounds created by insect feeding or other mechanisms; however, the bacterium does not enter the plant through natural openings or by seed transmission. Once in the host plant, the bacteria spread systemically to vines and other plant parts via the xylem. Vessels in the xylem become clogged with bacterial growth known as “gum,” which leads to wilting. Beetles lay eggs in the soil and the hatched larvae feed on the roots of the cucurbit plants. The larvae eventually become mature adults, feeding on infected plants and spreading the disease so that the cycle can continue. Recent research has shown that Erwinia tracheiphila moves both up and down the plant from the point of infection, and can be found in roots soon after infection.
(Left: Cantaloupe with Bacterial Ooze; Middle: Cucumber Stem with Bacterial Ooze; Right: Striped Cucumber Beetle)
Favorable Environmental Conditions
Temperature and humidity do not influence the development of this pathogen; however, the activity of the pathogen is enhanced during periods of dry weather or after intense rain (Blancard et al., 2005). Other favorable environmental conditions are those that aid in the over-wintering, feeding, and reproduction of the vector. This includes higher winter temperatures and good snow cover followed by a warm March and April.
Often Confused With
- Squash vine borer damage – Plants begin to wilt and eventually collapse, if not treated, which closely resembles the symptoms caused by Erwinia tracheiphila; however, squash vine borers cause very different damage than the cucumber beetles. Small borer holes near the base of the plant contain a green to orange sawdust-like material (insect frass) inside the stems.
- Fusarium root rot – Plants begin to wilt; however older leaves are usually the first to exhibit symptoms, followed by the younger foliage. The foliage becomes very chlorotic. There is also some vascular discoloration, but this discoloration appears as long brown streaks.
Apply management strategies for cucumber beetles on seedling cucurbits based on the table below or find more information at this link (Welty and Gardiner, 2013): Cucumber Beetle PDF
- Insecticides to Control Insect Vector – The main way of managing this disease is to use insecticides to control the vector. Some insecticides that have proven effective are permethrin, neem oil, carbaryl, insecticidal soap and pyrethrin-combination products. This is especially useful for young susceptible plants and during the months of April, May and June, when the beetles are laying eggs.
- Sound Cultural Practices – Roguing (removing) infected plants can reduce the progress of the disease (Latin, 2000). Eliminate weeds and volunteer cucurbits (Kurowski et al., 2015).
- Perimeter Traps – A perimeter trap crop (PTC) strategy has been emphasized as an alternative strategy. It consists of planting a border crop to intercept incoming beetles, reducing damage to the main crop. Even though no differences were found for incidence of bacterial wilt, this strategy can reduce the number of insecticide applications in the main crop by focusing sprays on the trap crop and spraying the main crop only when beetle thresholds are exceeded. Research in Iowa and Ohio with PTC reduced three to four insecticide applications per season (Baysal-Gurel et al., 2012a; Sun et al., 2014).
- Row Covers – The use of row covers has also the potential to protect plants from cucumber beetles. Row cover reduced bacterial wilt incidence in Ohio and Iowa studies (Baysal-Gurel et al., 2012b; Torres et al., 2012). This is particularly effective for parthenocarpic cucumber varieties that do not require pollination. In the case of other cucurbits, the row covers must be removed after plants begin to flower.