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Symptoms and Signs
Bacterial fruit blotch (BFB) affects the foliage at all growth stages and fruit of a wide range of cucurbitaceous hosts. Symptoms can be initially observed on cucurbit seedlings, between five and eight days after planting; depending on the environmental conditions. For most cucurbits initial seedling symptoms include water-soaking on the undersides of cotyledons (Figure 2). High relative humidly during early morning hours can lead to natural water congestion of cotyledons that can mimic BFB seedling symptoms. However, unlike natural water congestion, BFB-associated lesions have a greasy appearance and persist under dry conditions, i.e., observable after mid-day (Figure 3). Water-soaked lesions start as discrete spots but then coalesce and extend along the veins of cotyledons. Lesions can extend along stems to tissues of true leaves and in severe cases they can cause seedlings to collapse and die in a "damping-off fashion." Water-soaked lesions eventually dry to form elongated, dark to reddish-brown lesions that develop on and along cotyledon veins (Figure 4). Similar symptoms are produced on melon (Figure 5) and other cucurbit seedlings.
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On mature watermelon foliage, distinct dark to reddish-brown lesions develop along leaf veins; however, these may be inconspicuous and indistinguishable from symptoms of other foliar diseases (e.g., anthracnose and gummy stem blight). On cantaloupe leaves, symptoms include tan to reddish-brown lesions along leaf veins (Figure 6), but also include v-shaped lesions that extend from the margin to the base of the leaf (Figure 7). BFB symptoms on pumpkin foliage also include elongated tan lesions along leaf veins (Figure 8), but extensive chlorosis may be present (Figure 9). Although symptoms include leaf spots, these may be inconspicuous and have little to no effect on the host plant. The economic loss results from the fruit rot phase of the disease.
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BFB symptoms on watermelon fruits appear just prior to harvest maturity as small (< 1 millimeter in diameter), irregularly-shaped, olive-colored spots on the upper surfaces of the fruit (Figure 10). These lesions, although initially small, can spread and cover the entire upper surface of the melon. At early stages of infection these lesions are firm and rarely penetrate into the flesh of the fruit. Brown cracks may develop in the rind lesions and they may release amber-colored or effervescent ooze (Figures 11 and 12). At advanced stages, fruits collapse into a watery rot due to invasion by secondary colonizing organisms (Figure 13). BFB symptoms on melon fruit also start as discrete dark green spots (Figure 14); however, as fruit mature, lesions become small sunken depressions in the rind (Figures 15 and 16). In melons that develop a netted rind (e.g., cantaloupe), the netting fails to develop over necrotic areas, resulting in smooth sunken spots (Figure 17). Melon lesions do not expand on the fruit surface, but penetrate through the pericarp (fruit wall) to cause rotten brown cavities (Figure 18). Similar symptoms occur on pumpkin including water-soaked lesions with cracks in the rind (Figure 19) and internal fruit rot (Figure 20).
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