Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
This has been a very hot dry summer so far and we would expect to see pests such as twospotted spider mites and their damage to be common, which we have. However, I have been surprised by the amount of worm (Lepidoptera larvae) damage in tomatoes. Usually worms are a problem in a few fields every summer where they do some damage, but the amount of damage they have done in some fields this year is much greater, around 15-20% of harvestable fruit in several instances. The biggest culprit seems to be yellow striped armyworm (YSAW) Spodoptera ornithogalli (Fig. 1). As the name implies the larvae have two bright yellow stripes on the upper part of the worm running the length of its body. The yellow stripe is often flanked towards the inside with black triangular-shaped markings. This worm species tends to feed on the foliage of many plants, but most of the damage I have seen this summer has been on the fruit with little feeding on the foliage. The fruit damage usually appears as surface feeding (Fig. 1) or feeding holes that are very shallow and do not penetrate too deeply into the fruit (Fig. 2). This often leads to a dry type of damage as opposed to the smaller, deeper holes that often lead to a wet rot (Fig. 2). The YSAW overwinters as pupa in the soil and becomes active in late May or mid-June in our area. This year it has become active much earlier than it normally does and has built its population earlier too. We usually do not see this much damage until late August. Management must take place early when larvae are small; once larvae become large they are difficult to control.
Figure 1. Yellow striped AW and feeding damage on tomato
Figure 2. Yellow striped AW damage to ripening tomato fruit. Dry (yellow arrows) and wet damage.
Another surprise is that bacterial diseases are turning up in many tomato fields. Moist weather and splashing rains are most often needed for spreading bacteria. Maybe the presence of bacteria in the field is not too surprising, but what is surprising is the widespread nature of the bacterial spot, speck and sometimes canker diseases. Most tomato fields I have looked at in the last two weeks seem to have at least some if not a considerable amount of bacterial disease, usually on the lower leaves (Fig. 3) that in some cases has moved up to the pedicels of the fruit (Fig. 4). Infection of the flower or pedicel with bacterial spot is serious, causing early blossom drop (Fig. 5). From the pedicel the next stop for the bacteria, after a heavy thundershower, will be the fruit. A weekly mixture of mancozeb plus fixed copper or ManKocide should help with bacterial spot or speck, but once in the field, bacterial diseases are difficult to control. If a grower has an older tomato field that has bacterial spot in it that field should be plowed under as soon as possible as it will act as a nursery for spreading the disease to the younger tomato fields.
Figure 3 Bacterial spot or speck on tomato
Figure 4. Tomato pedicels and blossoms with bacterial spot
Figure 5. Blossom drop due to bacterial infection (yellow arrows) and the start of a flower being aborted (red arrow).
The last ‘surprise’ pest has been the seemingly sudden appearance of downy mildew in cucumber fields (Fig. 6). This disease usually needs cooler weather that we have had little of this summer. But on the 21 of July we had a cool wet period when several areas in the mid-Atlantic set a record low for the daily high (77o F). Right after this brief cool down the downy mildew seemed to explode. Many of the cucumber fields I visited in southern and central Maryland that had been harvested at least once had downy mildew. Once it starts it can defoliate a patch of cucumbers very quickly leaving any fruit to sunburn (Fig. 7).
Figure 6. Downy mildew on cucumber leaf
Figure 7. Cucumber plants defoliated due to downy mildew resulting in sunburned fruit