Many vegetable growers are closing in on the final harvest. Several growers have asked me about fungicide applications late in the season. In this article, I want to address when to stop. To limit the scope of this article, I will concentrate on tomato, cantaloupe and watermelon crops. These are crops where the fruit is consumed, not the foliage. For most vegetable crops, there is no need to apply a fungicide shortly before the final harvest. Foliage needs to be protected to preserve fruit quality. A plant with reduced foliage will produce a smaller fruit and/or fruit that have fewer sugars and other desirable compounds. I don’t know how much foliage needs to be reduced to affect fruit size or quality. However, I do know that for many foliar diseases, symptoms will not be obvious for a week to 10 days. It will take even longer for the foliar disease to significantly reduce foliage. Therefore, for[Read More…]
Dan Egel
Winter squash – butternut, acorn, and kabocha – in our downy mildew sentinel plot at Pinney Purdue were showing some wilted and stunted plants by late July (Figure 1). They are easily pulled up, the stem breaking off at ground level, revealing a brown stringy decayed-looking stem base (Figure 2). Sometimes there is a little whitish or maybe pinkish mold on the stem. I cut open a kabocha squash to look for squash vine borer larva and found sap beetles that seemed to be feeding inside the stem, but no vine borer (Figure 3). The sap beetles were clearly taking advantage of an opportunity, but not the cause of the wilt. Perhaps a borer had already come and gone. I used scotch tape to pick up some of the mold and put it on a slide to look at under the microscope. At 100X and 400X I saw among the[Read More…]
Downy mildew of watermelon has been observed on watermelon in Knox County in southwestern Indiana. Downy mildew of cucurbits has also been reported in southwestern Michigan on the Indiana border and central Missouri. All cucurbit growers in Indiana should be scouting and managing for downy mildew. The organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana because it requires living plant tissues. That means that the fungus-like organism that causes downy mildew has to be blown in every year. It is common for downy mildew to start the season in the Gulf States and migrate north with the cucurbit crops. Downy mildew apparently overwinters in northern Michigan/southern Ontario in greenhouses where cucumbers are grown year-round. Therefore, downy mildew is often found in Michigan before it is found in Indiana. On pumpkin and cucumber, downy mildew causes angular yellow lesions on leaves (Figure 1). Lesions on cantaloupe and watermelon tend to be diffuse[Read More…]
Recently, I have had a few phone calls about Plectosporium blight on pumpkins. This disease can be difficult to describe in words. However, once observed, Plectosporium blight is easy to remember. Therefore, this article will include photos of the disease. Lesions of Plectosporium blight are most often observed on the stems of affected plants. The lesions are small and irregularly shaped. The lesions often coalesce to form a scabby area (Figure 1 and 2). When the handle of the pumpkin is affected, the marketability of the pumpkin is affected. In severe cases, the pumpkin itself may have lesions of Plectosporium blight. Plectosporium blight lesions on fruit may be confused with bacterial spot. However, bacterial spot lesions are usually larger than Plectosporium blight lesions and do not coalesce over large areas like Plectosporium blight. This disease may be managed through a combination of cultural and fungicide treatments. Crop rotations[Read More…]
While cantaloupe and pumpkin growers are used to combating powdery mildew in Indiana, watermelon growers may not be familiar with the disease. Occasionally, I observe this disease on watermelon as well. If left uncontrolled, this disease can cause loss of foliage, loss of yield and lower quality fruit. This article will discuss the biology and management of powdery mildew of cucurbits with an emphasis on watermelon. While powdery mildew often causes a white talc-like growth on either side of the leaf, on watermelon the symptom may show up as a chlorotic lesion on the upper side of the leaf (Figure 1). The talc-like growth on the lower side of the leaf may be more idiffuclut to observe than on other hosts. Occasionally, powdery mildew may be observed on watermelon fruit (Figure 2). This article https://vegcropshotline.org/powdery-mildew-symptoms-vs-variegated-leaves/ has additional information about powdery mildew symptoms. The fungus that causes powdery mildew, Podosphaera xanthii,[Read More…]
Here in Indiana, whitefly problems are rare, but when encountered it is most often in protected ag production (greenhouse or high tunnel) and less often in the field. However, this is the time of year that you may be seeing them in either environment. Whiteflies are not true flies, but rather Hemipteran insects, more closely related to aphids and plant hoppers. They are sap sucking insects that feed on the phloem of the plant, making them efficient vectors of plant pathogens. Whiteflies produce honeydew secretions which can attract other insects or host the growth of sooty mold on infested leaves. There are two main species of whiteflies that may be encountered in Indiana: the greenhouse whitefly (Trialeurodes vaporariorum) and the sweetpotato whitefly (Bemisia tabaci; Figure 1). They can be distinguished by the way in which they hold their wings when at rest on the plant: sweetpotato whiteflies hold their wings[Read More…]
I have observed this disease in scattered commercial pumpkin and squash fields across Indiana. Symptoms: Bacterial spot causes ⅛-¼ inch angular leaf lesions that are white to light brown in color (Figure 1). These leaf lesions may be accompanied by yellowing (chlorosis). The more important symptom are the lesions on fruit that are scabby to raised, round and a light brown in color. These lesions are often less than ⅛ inch in diameter and do not extend into the surface of the fruit. However, lesions may become secondarily infected in which case lesions can become an inch or more in diameter. Such lesions may grow into the flesh of the fruit (Figure 2). Any type of fruit lesion can ruin the marketability of the fruit. Biology: Leaf lesions, while unimportant economically, are important in diagnosing bacterial spot before fruit is present. This head start allows growers to begin preventive measures.[Read More…]
This article is modified from Darcy Telenko’s article about field corn in a recent Purdue Pest and Crop newsletter. Southern rust of corn is normally a disease of tropical areas. During summer months, however, the fungus which causes southern rust, Puccinia polysora, may move into Indiana or other Midwestern states. Southern rust has officially been confirmed in Posey and Vigo County. If you think you have this disease contact me or submit a sample to the PPDL https://ag.purdue.edu/btny/ppdl/Pages/Submit-A-Sample.aspx Southern rust pustules generally tend to occur on the upper surface of the leaf, and produce chlorotic symptoms on the underside of the leaf (Figure 1). These pustules rupture the leaf surface and are orange to tan in color. They are circular to oval in shape. We are seeing a lot of common rust as well and both diseases could be present on a leaf. Common rust will form pustules on both sides[Read More…]
Cucurbit downy mildew has been observed on cucumber in the southwest corner of Michigan, just across the border from La Porte County and LaGrange Counties, Indiana. All cucurbit growers in Indiana should be scouting for downy mildew. Cucurbit growers in northern Indiana should be managing for downy mildew. The organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana. It has to be blown in every year. It is common for downy mildew to start the season in the Gulf States and migrate north with the cucurbit crops. Downy mildew apparently overwinters in northern Michigan/southern Ontario in greenhouses where cucumbers are grown year round. Therefore, downy mildew is often found in Michigan before it is found in Indiana. In most years, the downy mildew fungus will blow from southern Michigan to Ohio before it tracks south. Many cucumber varieties have some resistance to downy mildew. For susceptible cucumber varieties or[Read More…]
Vegetable growers are used to scouting for pests such as spider mites and aphids. Growers have come to recognize the yellow leaves caused by spider mites and the curled leaves caused by aphids. Growers understand that even after spider mites and aphids are dead, the symptoms of the damage may remain on the affected leaves. Whereas, a leaf distorted by aphids remains distorted even after the aphids are dead, a leaf with symptoms of a foliar disease such as anthracnose of watermelon or early blight of tomato typically contains viable spores even after repeated fungicide applications. So, what is the purpose of fungicide applications? Contact fungicide applications with active ingredients such as chlorothalonil (e.g., Bravo®, Echo®, Equus®, Initiate®) or mancozeb (e.g, Dithane®, Manzate®, Roper®, Penncozeb®) are used to coat the surface of the leaf so that when spores land on a leaf, they are neutralized. While it is true that[Read More…]
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