Earlier in August, downy mildew was reported on all cucurbit species in LaPorte County in northwest Indiana and on pumpkins in Starke County (just south of La Porte County). More recently, downy mildew was reported on cucumbers and butternut squash in Knox County in southwestern Indiana. In addition, downy mildew is strongly suspected on cucumbers in Jefferson County. Growers in nearby areas should take care to manage downy mildew if they have valuable cucurbit crops. However, this late in the season, it is unlikely that there will be widespread losses. Management of downy mildew of cucurbits is discussed in the Midwest Vegetable Production Guide for Commercial Growers 2017 mwveguide.org and in the extension bulletin Downy Mildew of Pumpkin https://www.extension.purdue.edu/extmedia/BP/BP-140-W.pdf. Note that downy mildew of cucurbits and downy mildew of soybeans are not caused by the same organism. Please call Dan Egel if you have questions or concerns.
Dan Egel
This time of year, I receive many complaints of pumpkin plants with yellow leaves. There can be many reasons why pumpkin plants have yellow leaves. The most common reason for yellow pumpkin leaves doesn’t have anything to do with a disease that can spread from plant to plant. Usually, the reason for the yellow pumpkin leaves has to do with lack of water, weather that has been too hot, nutrient deficiency or other stresses. The photos and discussion below will, I hope, illustrate my point. Let’s say you have a pumpkin field where you have pumpkin leaves that are yellow and you are wondering about the cause. You may want to ask yourself, which leaves are yellow and where are they yellow. In Figure 1, yellow pumpkin leaves may be observed. When one looks a bit closer to find out where the yellow leaves are, one can see that the[Read More…]
This disease was observed in southern Indiana recently. Cercospora leaf spot affects table beets and swiss chard. Symptoms include circular leaf spots that may have a reddish margin. The center of the lesions may start off a light brown and turn to gray after the fungus (Cercospora beticola) begins to sporulate. Under conditions conducive to disease, the lesions can coalesce and result in loss of foliage. Yield and quality of the crop can be reduced. Cercospora leaf spot is favored by rainy weather or overhead irrigation and temperatures from 77 to 95°F. The spores are readily dispersed in rainy, windy weather. Resistant cultivars are available. Fall tillage and crop rotations of 2 to 3 years should help to lessen disease severity. Several fungicides are listed in the 2017 Midwest Vegetable Production Guide for Commercial Growers including copper compounds, some of which may be allowed in organic certifications. Synthetic fungicides include:[Read More…]
Powdery mildew is a common disease of cantaloupe and pumpkin in Indiana. Occasionally, I observe this disease on watermelon as well. Recently, I have noticed more powdery mildew than usual on watermelon. 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. Powdery mildew is relatively easy to recognize; talc-like lesions occur on both sides of the leaf (Figure 1). (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, does not require leaf wetness for infection of leaves, only high humidity. The optimum temperature for disease development is 68 to 81°F. P. xanthii may survive for a period in crop residue as a resilient fungal structure, but the disease is so easily windborne, that crop rotation is not always[Read More…]
One of the most common diseases of pumpkin in Indiana is powdery mildew. Growers are naturally anxious to observe whether the fungicide they have been applying for this disease has been effective. Therefore, many pumpkin growers scout their fields for disease. Although powdery mildew is relatively easy to recognize, it is possible to become confused. Figure one shows two pumpkin leaves. The bottom leaf has the white, sporulation of the powdery mildew fungus in colonies randomly scattered across the leaf. The top leaf, has silvery coloration primarily along the vein. This latter leaf is a healthy variegated leaf. Some pumpkin varieties show this type of variegation more than others. It may be a good idea to study the photo shown here so that one can tell the difference between a pumpkin leaf with the disease powdery mildew and a healthy leaf that is merely variegated
Southern rust of corn is normally a disease of tropical areas. During summer months, however, the fungus which causes southern rust, Puccinia polysora, often moves into southern areas of the U.S. This summer, southern rust has been observed in at least 11 Indiana counties. Symptoms of southern rust include raised structures called pustules. If rubbed with a finger, the spores in rust pustules will come off leaving a stain on one’s hand. Southern rust develops rust pustules primarily on the upper leaf surface (Figure 1). Common rust typically has rust pustules on both upper and lower leaf surfaces. In severe cases, southern blight can cause rust pustules on ear husks and leaf sheaths. Late in the summer, dark pustules may be formed which are called telia. It is possible to confuse southern rust with other diseases, therefore, a confirmation by the Purdue University Plant and Pest Laboratory is advised. The[Read More…]
Southern blight of pepper and tomato thrives under hot, dry conditions. Usually, such conditions are more likely in August. Production under tunnels may contribute to the dry conditions that influences southern blight. This article will discuss the symptoms, biology and management of southern blight of tomato. Southern blight has a wide host range affecting many vegetable, field and ornamental crops. Tomato is the most important host. The disease is caused by the fungus Sclerotium rolfsii. This fungus is related to the one that causes white mold.The first symptom one is likely to observe of southern blight is plant wilt. At the base of the plant, one is likely to notice a canker with sclerotia that may be as large as a sesame seed (Figure 1). These sclerotia are survival structures for the fungus and allow the disease to occur in the same location years later. The sclerotia for southern blight are round,[Read More…]
I have observed this disease in scattered commercial pumpkin and squash fields across Indiana. Bacterial leaf spot of pumpkin is perhaps the most serious disease of pumpkin in Indiana today. 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 is 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[Read More…]
Manganese toxicity is a common problem for cantaloupes growing in sandy soils across southwestern Indiana. Because symptom of manganese toxicity can easily be confused with foliar diseases, growers may misdiagnose the problem and waste fungicides by spraying for nonexistent diseases. As we now know that manganese toxicity is a nutrient related disorder caused by low soil pH, it is important for growers to learn the symptom and address the problem in right directions. Manganese toxicity can develop on both cantaloupes and watermelons. But the symptom is more often observed on cantaloupes as they are more sensitive to acid soil conditions than are watermelons. The symptom on cantaloupes is first noticed when light green to yellow color shows between the veins on older leaves (Figure 1). Look at the leaves toward the sun and you will notice the chlorosis is formed by numerous tiny light green to yellow pin-hole type spots growing[Read More…]
I have observed very few foliar diseases of cucurbits this season. However, I have had several worried phone calls about these diseases, so here is an update. Alternaria leaf blight-this disease is caused by a fungus that survives in crop residue. It usually is not an important disease. However, Alternaria can cause brown lesions with a ring-like structure in them. This disease is more important on cantaloupe. I have not observed this disease in 2017. Anthracnose-after some effort, I have been able to start this disease in my own plots. Look for jagged, brown lesions on leaves and stems and pitted lesions on fruit. Except for my own experiments, I have not observed this disease in 2017. Gummy stem blight-this disease does not seem to occur as often as anthracnose. However, this disease showed up uninvited in my research plots. I have not observed this disease in commercial fields. The[Read More…]
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