Wenjing Guan

Vegetable Crops Hotline Editor & Clinical Engagement Assistant Professor
Department of Horticulture and Landscape Architecture
Area(s) of Interest: Commercial Vegetable and Melon Production
Wenjing Guan's website

106 articles by this author

Article List
Figure 1. Fertilizers form insoluble precipitations that clog drip emitters.

When mixing fertilizers, it is important to check fertilizer compatibility before application. If incompatible fertilizers are mixed, they form insoluble precipitations that can clog drip emitters and damage sprayers used to apply foliar fertilizers (Figure 1.). This article discusses a few scenarios for which special attention should be paid on the solubility of mixed fertilizers. Scenario 1. A grower is using 20-20-20, a complete fertilizer to fertigate tomatoes. To prevent blossom end rot, he decided to add calcium nitrate in his fertigation program. However, problems may be caused by the application of these two fertilizers.  The reason is that calcium from calcium nitrate and phosphate from ammonium phosphate in the complete fertilizer may form calcium phosphate, which is insoluble in water. Scenario 2. A soil test indicates that tomato plants are low in magnesium. Epson salt (magnesium sulfate) is recommended to correct magnesium deficiency. The grower should avoid applying Epson[Read More…]


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Southwest Purdue Ag Center High Tunnel Tour Date: Thursday, August 10, 7 p.m.-9 p.m. (EDT) Location: Southwest Purdue Agricultural Center, 4669 North Purdue Road, Vincennes, IN, 47591 Registration: Visit http://tinyurl.com/yc5lqvez or call (812) 886-0198 For more information, contact: Wenjing Guan at guan40@purdue.edu or Dan Egel at egel@purdue.edu During the evening event at Southwest Purdue Agricultural Center, attendees will see demonstrations of soil solarization, end-of-season clearance of soil covers, sprayers used for small-scale plots, and an innovative season-long low tunnel system for growing tomato and pepper. Attendees will also learn how to ID tomato diseases by walking in the field with a plant pathologist. You are also welcome to bring your own disease samples for identification. A NRCS representative will share the insights about high tunnel cost-share program. We will also discuss issues relating to how to choose, locate and make pre-construction decisions for a high tunnel. Pinney Purdue Vegetable and[Read More…]


The Indiana Pesticide Clean Sweep Project is a great opportunity to legally dispose of unwanted chemicals. It is free of charge up to 250 pounds. To dispose of pesticides, complete the 2017 Pesticide Clean Sweep Planning form (the form is attached to the newsletter) and mail, fax or e-mail the completed form to Kevin Neal at Purdue University. 175 South University Street, West Lafayette, IN 47907-2063, (765) 494-4331 (fax) or nealk@purdue.edu no latter than Monday, July 31, 2017. Then bring your labeled, leak free and safe to transport containers to the collection site. Do NOT mix materials. Pesticides will be accepted from 9:00 am to 3:00 pm local time at the following dates and locations: August 15, 2017: LaPorte County Fairgrounds in LaPorte, IN August 16, 2017: Tippecanoe County Fairgrounds in Lafayette, IN August 17, 2017: Grant County Fairgrounds in Marion, IN August 22, 2017: Franklin County Fairgrounds in Brookville, IN August 23, 2017:[Read More…]


After harvest, storing vegetables in optimal conditions is important to ensure the whole season’s hard work has paid off. This article discusses the optimum storage conditions for tomato, pepper, cucumber, watermelon, cantaloupe and sweet corn. Tomato Ideal storage conditions for tomatoes depend on the maturity stage of picking. If tomatoes are picked at mature green, store them in 66 to 70°F with 90 to 95% RH would encourage uniform ripening. Temperatures above 81°F reduce intensity of red color and reduce fruit shelf-life. Green tomatoes are chilling sensitive. If the temperature is below 55°F, fruit may develop chilling injury. Red tomatoes are safe to store at 50°F, however, flavor and aroma may be negatively affected compared to storing them at 55°F. Pepper Optimum storage condition for peppers is 45 to 55°F with 90 to 95% RH. Temperatures lower than 45°F may cause chilling injury. Colored peppers are in general less chilling[Read More…]


Figure 5. Breakdown and death of older cantaloupe leaves caused by manganese toxicity.

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…]


Figure 1. Initial symptom of blossom end rot on pepper.

In the past few weeks, we have received several reports about blossom end rot on tomatoes and peppers as the crops start to set fruit. Blossom end rot is a physiological disorder (not an infectious disease) that commonly occurs on tomatoes and peppers. Initial symptoms of the physiological disorder include dark green or brown water-soaked leisure occurring on the bottom of the fruit (Figure 1). The lesion then expands into sunken, leathery brown or black spots (Figure 2 and 3). In severe cases, the lesion can expand to half size of the fruit. The symptoms on tomatoes can be observed on fruit from fruit set to fruit the size of golf balls. Fruit on the same cluster tend to show symptoms simultaneously. On peppers, the symptoms are more likely appear during fruit expansion. The affected fruit often change color prematurely. Under moist conditions, opportunistic molds might develop on the affected tissues[Read More…]


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Purdue Extension and Indiana University are collaborating to offer a series of field days featuring high tunnels. The events will be hosted July 18 at the Meigs Horticulture Research Farm in Lafayette; Aug. 10 at the Southwest Purdue Agricultural Center in Vincennes; Aug. 15 at the Pinney Purdue Agricultural Center in Wanatah; and Sept. 27 at the Hamilton County Extension Office and Full Hand Farm in Noblesville. Meigs Horticulture Research Farm High Tunnel Field Day Date: Tuesday, July 18, 10 a.m.-1 p.m. (EDT) Location: Meigs Horticulture Research Farm, 9101 S. 100 E, Lafayette, IN, 47909 Registration: Visit http://tinyurl.com/yc5lqvez or call (765) 494-1296 For more information, contact: Lori Jolly-Brown at (765) 494-1296 or ljollybr@purdue.edu The field day at Meigs Horticulture Farm will focus on high tunnel production of cucurbit crops. It will feature tours of conventional and hydroponic high tunnel cucumber and melon production. The use of insect-exclusion screens to control cucumber[Read More…]


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Indiana Fruit & Vegetable Field Tour The tour will be held at Tuttle Orchards, 5717 North 300 West Greenfield IN 46140, June 28, 9:00 am -4:00 pm. On-site registration 9 am at Tuttle Orchard. Registration fee is $5 per person. For further questions contact Lori Jolly-Brown at ljollybr@purdue.edu or (765) 494-1296. Southwest Purdue Ag Center Field Day  Southwest Purdue Ag Center Field Day will be held on June 29, 2017 at Southwest Purdue Agricultural Center, Vincennes, IN. The field day provides two choices of tours: Horticultural Crop Production and Agronomic Crop Production. The Horticultural Crop Production tour will feature Organic Tomato Production, High Tunnel Vegetable Production, Grape Research, Protecting Pollinators while Managing Insect Pests in Watermelon Production, and Produce Food Safety. A meal will accompany the tour with PARP classes available after lunch. Please contact Barb Joyner (joynerb@purdue.edu, (812) 886-0198) for registration, or register online at https://purdue.qualtrics.com/jfe/form/SV_0rICQrMVJmiMnqZ. Organic Vegetable Seed Production & Varietal Selection Workshop  The workshop[Read More…]


Recent dry weather raises concern about effectiveness of preemergent herbicides.  Preemergent herbicides applied on the soil surface need to be moved into the soil where the target weed seeds are germinating (normally 1-2 inches deep) in order to be effective. The process normally requires 0.5 to 0.75 inch of water within a few days of herbicide application. For example, product labels state that Curbit 3EC® needs 0.5 inch of water within 5 days of application, and Dual Magnum® needs at least 0.5 inch water within 10 days of application. In nonirrigated areas where rain is not anticipated, a shallow cultivation is needed to move the herbicide into the zone of weed seed germination. Most areas of Indiana were dry the last week of May and first week of June. If preemergent herbicides were not incorporated in a timely manner by water or machine, unsatisfactory weed control might be observed. In that case,[Read More…]


Figure 1. A 30% black shade cloth was added to one of the high tunnels

Tomatoes growing in high tunnels are in the middle of or close to harvest. Developing and maturing fruit are under leaf canopies. On the top of the plants, many flowers are still blooming. These flowers will contribute to the second big harvest. Although tomatoes in June are most valuable, we certainly appreciate big, red and delicious tomatoes in July and August.  To ensure a sustained yield, it is important for these flowers to set fruit. The process of fruit set is very sensitive to excessively high temperatures. When temperatures rise above 100°F, even just for a few hours for a handful of days, tomato flowers may be aborted and fruit set fail. Night temperatures above 75°F may also cause tomato fruit set failure. In addition to fruit set, high temperatures affect fruit ripening process. Ethylene associated ripening decreases markedly at a temperature above 93°F. As a result, there might be an increasing number of[Read More…]