Published: August 3, 2017

Weeds: Glyphosate resistant weeds, like waterhemp, are increasing for vegetable growers around the Central Missouri Produce Auction. To address these weeds growers will have to try other herbicides. Having the most recent year's edition of the Midwest Vegetable Production Guide for Commercial Growers will be helpful, as changes to herbicide labels can occur. Glyphosate resistant weeds have become particularly troublesome for soybean growers and an outcome has been the development of soybeans resistant to dicamba. This summer was the first year these soybeans could be grown and sprayed with dicamba. The fact that this would likely occur in hot humid weather had many concerned, even with the new formulations of dicamba that were required, as dicamba is prone to moving off site in that kind of weather. The concerns were warranted and an excellent article, by Kevin Bradley (MU Weed Scientist), reviews what transpired thru July 'Ag Industry, Do We Have a Problem Yet?'. The next issue of mpg will delve into 2017 and accidental herbicide contamination of vegetables in Missouri.

Insects: Japanese beetles have been getting a lot of attention throughout the central part of Missouri. In past years, field corn and soybean growers often just sprayed the edges of their fields for control. This year many are spraying entire fields. Some vegetable growers reported enough pest pressure that both green beans and sweet corn required treatment. This is notable, as the vegetable production guide (mentioned above) does not list this insect as a pest for either. One grower reported the beetles attacking the tassel whorls such that he had to spray BEFORE he normally does for corn earworm. And then he had to shorten his spray interval to keep them off the corn silks, spraying every two days instead of at three days. A MU agronomy specialist, with scouting experience for this pest, said the beetles are especially attracted to silks at 4 to 5 inches long.

Diseases: Vegetable diseases have been common this year. The mild winter and heavy spring rains promoted disease in numerous types of crops. A list of diseases, diagnosed on vegetables are listed in Table 1. Disorders and pest issues are not listed.

Table 1: Diseases that were diagnosed more than once are noted with an asterisk (*).

The most notable disease has been Pythium root rot of watermelon. Pythium diseases on watermelon are typically seedling blights. We usually see these when seeds are planted in cold soils that become saturated, thus promoting damping off. Or we see Pythium root rot diseases on young plants that were recently transplanted prior to cool, wet weather. However, due to heavy spring rains and fluctuating soil temperatures Pythium root rot has been diagnosed twice. Both incidents have predispositioning factors that most likely led to the issue. One case, the plants are growing in a low area and heavy rains flooded the site twice. Both causing plants to be in standing water. The second case, the plants were not in standing water and in a well-draining area. However, the grower had worked in a lot of composted manure and straw. Therefore the site has high organic matter. Organic matter holds moisture in the soil, a necessary component for good growing conditions. But in certain situations you can have too much of a good thing. In this case, the high organic matter is keeping soils saturated and promoting Pythium.

Pythium is commonly referred to as a water mold, same as its "cousin" Phytophthora. Now, Phytophthora diseases on mature watermelon are quite common in wet years but we haven't seen any at the clinic this year. There are diagnostic differences between the two, besides the serological assay we run at the Plant Diagnostic Clinic for confirmation. Plants with Pythium have rotted roots, similar to Phytophthora, the visual difference is the discoloration is more of a yellowish to dark brown opposed to the red-brown / black discolor caused by Phytophthora (Picture 1a). Also the cortex tissues (epidermis), of a Pythium infected root, are slimy to the touch and remove easily (Picture 1b). When crowns are split, the Pythium infected plants have little discoloration, compared to Phytophthora, that has so far been limited to the lower crown and are the yellow to yellow-brown color (Picture 2). Recommendations are to manage water, don't over irrigate and create drainage in low areas. Also, remove and destroy infected plants but do not compost as the pathogen can remain active in the tissues. After plant removal, till the soil to break up any plant debris left behind and promote aeration. A rotation could be useful but Pythium species are hosted by many plants. Incidence and severity of Pythium diseases are determined by the weather and overall plant health. Thus, the growers should avoid planting too early, when soil temperatures are low, and minimize wounding when transplanting. Fungicides can be used preventatively, recommended products include, but not limited to, Agri-fos, Previcur Flex and/or Ridomil. Follow label instructions when applying pesticide products.

Picture 1: (a) Watermelon roots infected with Pythium are slightly discolored, lack fine roots and the cortex is easily removed and feels slimy to the touch. (b) Note area in the circle where cortex was removed by rubbing a finger along it gently.

Picture 2: Watermelon crowns have slight discoloration due to Pythium rot. Rot is in the lower crown as noted by circle.