Volume 26, Issue 13 June 22, 2018 Vegetable Crops Downy Mildew Resistant Cucumbers – Gordon Johnson, Extension Vegetable & Fruit Sweet Corn Insect Management – David Specialist; [email protected] Owens, Extension Entomologist, With downy mildew being found in pickling [email protected] and Bill Cissel, Extension cucumbers on Delmarva, resistant varieties Agent – Integrated Pest Management; should be considered. [email protected] We now have several slicing and pickling Sweet corn trapping data is updated by Tuesday cucumbers with some degree of resistance to the and Friday mornings and can be accessed here: current aggressive cucumber downy mildew http://agdev.anr.udel.edu/trap/trap.php . I strain. According to Clemson University new expect earworm counts to start decreasing; you research has identified two strains of cucurbit can look at our historical trapping data downy mildew: A1 attacks cucumber and Trap BLT – CEW Pheromone Corn cantaloupes (the most aggressive strain), while Location (3 nights CEW Spray A2 attacks watermelon, pumpkins, and squashes. total (3 nights Schedule In gynoecious slicing cucumbers, Bristol, catch) total catch) SV3462CS and SV4719CS and associated Dover 1 8 4 day pollenizers have intermediate resistance to Harrington 0 4 4-5 day cucumber downy mildew. In gynoecious pickles, Milford 0 3 5 day Citadel, Peacemaker, and SVCN6404 with Rising Sun 0 22 4 day associated pollenizers have intermediate Wyoming 0 0 No spray resistance. Next generation downy mildew Bridgeville 0 12 4 day resistant varieties are being tested this year on Concord 0 3 5 day Delmarva. Georgetown 0 0 No spray These varieties are not immune to downy Greenwood 0 0 No spray mildew, but they show less disease under the Laurel 0 7 4 day same pressure and will out yield susceptible Seaford 0 0 No spray varieties. Under disease pressure they perform The spray schedule listed in the table is based best with a fungicide program using downy off our sweet corn action thresholds which can mildew materials. Research is being conducted be found here: to see if there is the potential to reduce http://extension.udel.edu/ag/insect- fungicide applications. management/insect-trapping-program/action- thresholds-for-silk-stage-sweet-corn/ . Shown in the table below is the yield of pickling cucumber varieties with and without intermediate resistance to cucumber downy June 22, 2018 Weekly Crop Update Volume 26, Issue 13 1 mildew with one fungicide application for downy Lack of fruit set can result from a lack of mildew under moderate disease pressure. Note pollination due to reduced bee activity, reduced that the resistant varieties significantly pollen viability, or reduced pollen germination in outperformed the susceptible variety high heat. As an example, a female watermelon (Georgetown, DE, 2017). flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a Variety Yield minimum of 8 visits by a honey bee. Research Bu/a has shown that over 20 visits may be required to SVCN6404 + SV2987CL pollenizer 329 a achieve full set and full size in some cucurbits. (Resistant) Citadel + SV2987CL pollenizer 276 b This year (2018), early fruit set in watermelon (Resistant) and cantaloupe may be off due to poor weather Vlaspik + Sire pollenizer 214 c during early flowering. Bees flights are reduced (Susceptible) significantly in rain and when winds are 15 mph p 0.001 or greater. Cloudy weather also reduces bee activity. Bees also do not fly much below 55°F, so on cold mornings, as we often had in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon. In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather vine crops closest to the hives will have the best set and furthest from the hives will have the worst. This year another problem is that some watermelon fields have lost significant numbers of pollenizer plants due to poor weather conditions during or after planting. This means Downy mildew pressure with susceptible and that pollen will be limiting. Research has shown resistant varieties planted side by side in a trial that were pollen is limiting, fruit numbers will located on the Eastern Shore of Maryland in be reduced with distance from a pollen source. 2018. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing. Fruit Set and Fruit Carry in Cucurbit Vine Another common question from growers and Crops – Gordon Johnson, Extension Vegetable & crop consultants is how many fruit should a plant Fruit Specialist; [email protected] carry and what will affect fruit “carry” in vine Watermelons and cantaloupes are setting fruit, crops. pickle harvest is underway, summer squash is in For watermelons, a healthy, vigorous plant may full production and pumpkin planting is finishing set 3-7 fruits initially. However, for mid-size and up. With the many rainy days as well as high larger watermelons, the plant will only carry 2-4 winds and storm damage this season, fruit loads fruit at any time. Smaller fruited varieties will in some early planted vine crops has been more fruits per plant but essentially the same affected and higher numbers of quality defects total pounds as larger types. This is the carrying are evident. capacity of the plant and is directly related to June 22, 2018 Weekly Crop Update Volume 26, Issue 13 2 the quantity of photosynthates being produced harvested as are watermelons. Medium sized by the plant, mostly in the leaves. Any Jack-o-lantern types will carry 1-2 fruits, larger additional fruits, even if initially set, will be types closer to 1. All others will be aborted. aborted. Once the first fruit ripens and is Smaller types will carry more depending upon harvested, additional sets can be carried. To their size in pounds (for example a variety with 5 carry the maximum amount of fruit, it is lb average will carry 4-7 fruits). Maximum necessary to maintain high plant vigor and good carrying capacity in pumpkins is largely affected foliage health. This requires paying close by variety (varieties with some heat tolerance attention to irrigation and fertility programs; will carry more fruits in our climate) and foliage having excellent disease, insect, and mite health. Excess nitrogen fertilization will often control; and having good pollinator activity delay fruit set in pumpkins. during pollination and fruit set. In watermelons, if average fruit carry is less than 2 per plant, this In gynoecious cucumbers grown for once over is a sign that the plants have reduced vigor and pickle harvesting, there will be two fruits set on are under stress. Repeated fruit set depends on adjacent nodes that are ready for harvest at any maintaining vine health through the season. one time. These will be set on nodes 2-6 commonly. The pollinizers that make up a small Another factor to consider is where fruit set is percentage of the population will set pickles occurring. Crown sets are desired in every fifth node generally and therefore only watermelons, especially in early plantings. one fruit will be ready for harvest. Yield Crown sets are those that occur on nodes closest reductions in gynoecious pickling cucumbers to the base of the plant, within the first 8 nodes. occur when there is a loss of set so that fruits Having good crown sets requires that plants have are not on adjacent nodes. Parthenocarpic good early growth so that adequate leaf area is pickle varieties that set fruit without pollination produced that can support early set fruit as well will commonly have 3-5 pickles on adjacent as proper pollination (sufficient bees). Lack of nodes ready for harvest at any one time. This crown set is a sign of poor early growth, early allows them to be planted at much lower plant stress, or of problems with pollination. densities. Growers with early-planted watermelons this year (those planted the last week in April or first 2 weeks in May) are likely to see reduced crown Tomato Pith Necrosis – Jerry Brust, IPM set and may see increased numbers of seedless Vegetable Specialist, University of Maryland; melons with defects such as distinct lobes [email protected] (noticeably triangular) or hollow heart and standard seeded pollenizers with pinched ends. In the last week, tomatoes from different These are signs that pollination was lacking counties in Maryland were found with the same during early fruit set. This can occur when there unusual disease symptoms, called tomato pith is a lack of pollen – pollenizers have not necrosis. All the problem tomatoes were from produced enough male flowers or are delayed in early planted fields. Tomato pith necrosis is producing male flowers. In some years, fields caused by the soilborne bacterium Pseudomonas have had losses of pollenizers, due to the poor corrugata. It has occurred infrequently in weather during transplanting, requiring Maryland over the past few decades. The disease replanting. This may reduce pollen for the first usually is found in early planted tomatoes when set female flowers in triploids, reducing crown night temperatures are cool, but the humidity is sets. Reduction in bee activity during the stormy high, and plants are growing too rapidly because weather this June may also reduce early sets. of excessive nitrogen application. We have had a Early plant stress such as the wind damage, spring with some cool nights in May and high flooding, and hail damage we have seen this humidity with all the rain we had.