Posts Tagged ‘watermelon’

Tissue Testing and Petiole Sap Testing for Vegetables

Friday, May 18th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recommended fertility programs for vegetable crops are given in the Commercial Vegetable Production Recommendations publication for Delaware and surrounding states. See http://ag.udel.edu/extension/vegprogram/publications.htm for an electronic version.

While these recommendations should be the base of a fertility program, additional monitoring of plant nutritional status is recommended, especially for highly managed crops such as those grown in plasticulture where fertilizers can be injected through the drip irrigation system.

Tissue testing involves taking samples from the plant at various times during the growth period, most commonly leaves, and sending them to a laboratory for mineral nutrient analysis. Petiole sap testing involves taking leaf petioles and expressing the sap which is then tested for nitrate and/or potassium using portable meters.

When taking tissue samples specific procedures should be followed to obtain reliable results. The following are recommendations from the University of Florida.

“The sample is a whole leaf sample and it should not contain any root or stem material. For sweet corn or onions, the leaf is removed just above the attachment point to the stalk or bulb. For compound leaves (carrots, peas, tomatoes, etc.), the whole leaf includes the main petiole, all the leaflets and their petioliules. For heading vegetables, it is most practical to take the outermost whole wrapper leaf. When sampling particularly young plants, the whole above-ground portion of the plant may be sampled.”

Most commonly the most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color.

“A proper leaf sample should consist of about 25 to 100 individual leaves. The same leaf (i.e., physiological age and position) should be removed from each sampled plant. Plants damaged by pests, diseases, or chemicals should be avoided when trying to monitor the nutrient status of the crop. Individual plants, even side-by-side, may have a considerably different nutrient status. Therefore, by sampling a sufficiently large number of plants, the error due to this variability can be minimized. More accuracy in determining the actual nutrient status is derived from a larger sample size.”

“Samples are often contaminated by fungicides, nutrient sprays, soil, or dust. Data obtained from contaminated leaf samples will be misleading. Decontamination of some dust or soil is best accomplished by quickly rinsing in a dilute non-phosphate detergent solution (2%) followed by two distilled water rinses. Tap water should not be used because it can be high in certain nutrients such as Ca, Fe, Mg, or S. Leaf samples should be washed quickly to minimize the leaching of certain nutrients (especially K) from the leaves.”

“Following rinsing, the sample should be blotted dry with absorbent paper. The samples should be air-dried for several hours before shipment. If a plant analysis mailing kit is not available, the samples should be wrapped in fresh absorbent paper and placed in a large envelope (plastic bags must not be used). The sample should be shipped or delivered immediately to the soil and plant analysis laboratory. An air-dried sample, if loosely packed to avoid rotting, will last two to three days before decomposition begins.”

“If the samples must be held for any length of time before shipping, they should be dried at 150°F in a ventilated oven (leave the door ajar) until dry weight is constant. Once dried, the sample can be placed in a plant analysis mailing kit or a large envelope. This ensures the integrity of the sample until shipping is possible.”

Petiole sap testing is useful for monitoring nitrogen and potassium and can give very quick results with the use of portable meters. The following are guidelines for petiole sap testing from the University of Florida:

“For sap testing, petioles collected from most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color. A random sample of a minimum of 25 petioles should be collected from each “management unit” or “irrigation zone.” Management units larger than 20 acres should be subdivided into 20-acre blocks. Leaves with obvious defects or with diseases should be avoided. Sampling should be done on a uniform basis for time of day (best between 10 AM and 2 PM), and for interval after rainfall or fertilization.”

“Whole leaves are collected from the plant and the leaf blade tissue and leaflets are then stripped from the petiole. A petiole of several inches in length remains. Petioles are chopped into about one-half inch segments. If analysis is not to be conducted immediately in the field, then whole petioles should be packed with ice and analyzed within a few hours of collecting. Given more extreme environmental field conditions (high temperature and bright sun), more dependable results are obtained by making measurement in the lab or office than outdoors.”

“Chopped petiole pieces are mixed and a random subsample (about ¼ cup) is crushed in a garlic press, lemon press, or hydraulic press (obtainable from HACH Co., Table 4). Expressed sap is collected in a small beaker or juice glass and stirred.”

Follow the instructions for the specific meter you are using to analyze the sap. If sap has too high of concentration of nitrate or potassium for your meter, then you will need to dilute the sap to conduct the test.

Information on tissue testing and petiole sap testing for vegetables including tables with recommended levels at different growth stages can be found at this site http://edis.ifas.ufl.edu/ep081.

The following are recommended values for watermelons: 

Plant Tissue Macronutrient Ranges for Watermelons at Different Growth Stages

Crop Plant Part Time of Sampling Status - – - – - – - – - – % – - — – - – - -
N P K Ca Mg S
Watermelon MRM* leaf Vining before flowering Deficient <3.0 0.3 3.0 1.0 0.25 0.2
Adequate range 3.0 0.3 3.0 1.0 0.25 0.2
4.0 0.5 4.0 2.0 0.5 0.4
High >4.0 0.5 4.0 2.0 0.5 0.4
Toxic (>) - - - - - -
MRM leaf First flower Deficient <2.5 0.3 2.7 1.0 0.25 0.2
Adequate range 2.5 0.3 2.7 1.0 0.25 0.2
3.5 0.5 3.5 2.0 0.5 0.4
High >3.5 0.5 3.5 2.0 0.5 0.4
MRM leaf First fruit Deficient <2.0 0.3 2.3 1.0 0.25 0.2
Adequate range 2.0 0.3 2.3 1.0 0.25 0.2
3.0 0.5 3.5 2.0 0.5 0.4
High >3.0 0.5 3.5 2.0 0.5 0.4
MRM leaf Harvest period Deficient <2.0 0.3 2.0 1.0 0.25 0.2
Adequate range 2.0 0.3 2.0 1.0 0.25 0.2
3.0 0.5 3.0 2.0 0.5 0.4
High >3.0 0.5 3.0 2.0 0.5 0.4

*MRM – most recently matured leaf with petiole.

Petiole Sap Nitrate and Potassium Concentration Ranges for Watermelon

Crop Stage of Growth Fresh Petiole Sap Concentration (ppm)
K NO3-N
Watermelon Vines 6-inches in length

Fruits 2-inches in length

Fruits one-half mature

At first harvest

4000 to 5000

4000 to 5000

3500 to 4000

3000 to 3500

1200 to 1500

1000 to 1200

800 to 1000

600 to 800

 

MELCAST Fungicide Scheduling for Watermelon

Friday, May 18th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Gummy stem blight (caused by Didymella bryoniae) is the most important foliar disease of watermelon in Maryland and Delaware. The disease affects leaves (Fig. 1), stems and vines of watermelon, resulting in fewer and smaller fruit. Anthracnose (Fig. 2), which is caused by Colletotrichum orbiculare, also occurs yearly. In MD and DE, yield losses due to gummy stem blight and anthracnose of 20 to 100% would occur in the absence of effective fungicidal control.

Beginning in 1997 the DE IPM, MAR-DEL Watermelon growers, Maryland Vegetable growers, and other grant funds have supported dissemination of a weather-based fungicide application program for watermelons, Melcast. Melcast is a weather-based spray advisory program for watermelon developed at Purdue University. The program uses hours of leaf wetness and temperature during leaf wetness periods to determine when a fungicide should be applied. In MD and DE, we have slightly modified Melcast so that fungicides are scheduled earlier. As a result, fungicides scheduled by Melcast, successfully manage anthracnose and gummy stem blight. Growers that use Melcast report reducing their fungicide applications by two per season compared to standard schedules. Six research trials were conducted over three years in our region to evaluate Melcast. In four of those trials yield was the same when sprays were applied according to Melcast compared to weekly applications. In one trial yield was higher, and in one trial yield was lower, when sprayed according to Melcast in comparison to weekly sprays.

Since our original trials of Melcast, several newer and highly effective fungicides have been registered for gummy stem blight and anthracnose. We are testing Melcast again with these effective materials. To use Melcast on your farm, please call Karen Adams at (302)856-7303 and give us your name and Fax number or e-mail address. More details about how the program works are available at our Disease Forecasting Web page: http://mdvegdisease.umd.edu/forecasting/index.cfm

Figure 1. Large dark brown foliar lesions of gummy stem blight

Figure 2. Anthracnose lesions on cucurbits are angular in appearance. Tiny black spots can be seen through a hand lens in the tan centers of lesions.

Transplant Shock

Friday, May 11th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A number of watermelon fields have had issues with poor plant performance and plant losses after transplanting in the last 10 days. Transplant shock is most prevalent when there are cold, windy conditions after transplanting and when night temperatures drop below 50°F. Plant and planting conditions that increase the risk of plant shock include:

Poor hardening off. Plants that come directly out of greenhouses or that have just recently come out of houses are most at risk. A proper hardening off will include reducing fertilizer and water and exposing plants to outside conditions in a protected area. It takes a minimum of 5 days to harden off plants.

Different plant maturities. Younger plants are more susceptible to shock. In watermelons, pollenizers are often younger than seedless due to having more rapid growth. Pollenizers are often most susceptible to plant shock after transplanting.

Small root systems. Plants grown in small cell sizes have fewer roots and if rooting conditions after transplanting are not favorable, they will be at a higher risk of shock than plants with larger root systems.

Root bound plants. An opposite problem can occur where plants have been in trays too long and roots have become root bound. Root bound plants dry out more quickly and often do not send out new roots as quickly because many roots in the root ball have died or are growing in circles in the cell.

Root systems not fully formed. In cells of plant trays, if the plant has not produced sufficient roots, it will not pull out of the tray properly and roots will be damaged when extracting plants and plants will be more susceptible to shock.

Rough handling during transplanting. If transplant crews damage plants when pulling out of trays and when setting plants, there will be increased plant shock. This includes stem crushing or damaging roots when extracting plants.

Setting plants too low or too high. In the transplanting process, burying plants too deep where green stem or leaf tissue is below ground can lead to that tissue being exposed to rotting organisms. Conversely, if root systems exposed (set to high), they can dry out and cause plant loss.

Inadequate plant water. If there is inadequate water at transplanting, plants can dry out and losses can occur.

Too much fertilizer. Too much fertilizer in the transplant water or in beds near the plant can cause salt injury and plant losses.

Poor plant handling. Keeping plants in tight conditions such as plant trucks for long periods of time, in extreme heat conditions, or where they have no light for an extended period will weaken plants and when exposed to the direct sunlight after transplanting, losses can occur. Plants shipped in that have been in transit too long or where truck conditions were stressful (cold or hot) will have more risk of shock. Plants that have dried out before transplanting are also at risk.

MELCAST for Watermelons

Friday, May 11th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

The weather based forecasting program MELCAST on watermelon will begin next week. MELCAST is a weather-based spray scheduling program for anthracnose and gummy stem blight of watermelon. If you received a report in 2011, you should automatically receive the first report next week. If your email or fax number has changed, please call us. If you do not receive a report and would like to, please call Karen Adams at (302)856-7303 and give us your name and Fax number or e-mail address. MELCAST also is available online – bookmark the site http://mdvegdisease.umd.edu/forecasting/index.cfm. Click on the watermelon picture.

To use MELCAST for watermelons, apply the first fungicide spray when the watermelon vines meet within the row. Additional sprays should be applied using MELCAST. Accumulate EFI (environmental favorability index) values beginning the day after your first fungicide spray. Apply a fungicide spray when 30 EFI values have accumulated by the weather station nearest your fields. Add 2 points for each overhead irrigation that is applied. After a fungicide is applied, reset your counter to 0 and start over. If a spray has not been applied in 14 days, apply a fungicide, reset the counter to 0 and start over. Please call if you have any questions on how to use MELCAST on your crop (Kate Everts at 410-742-8789).

Do not use Quadris, Cabrio or Flint on watermelons in Maryland or Delaware. Under low disease pressure, use Chlorothalonil (Bravo, etc.) applied according to MELCAST. Under high disease pressure alternate chlorothalonil with Pristine plus chlorothalonil, Folicur plus chlorothalonil, Inspire Super plus chlorothalonil or Luna Experience plus chlorothalonil applied according to MELCAST. If a severe disease outbreak occurs in your field, return to a weekly spray schedule.

Cucurbit Downy Mildew Fungicide Decisions

Friday, May 11th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Downy mildew on cucurbits has been a problem on Delmarva beginning in early July for the last few years. Good fungicides for management are available. However, last year in my trials, one of these fungicides, Presidio, was not as effective as expected. Looking ahead to your spray program, be careful not to rely on one fungicide class. It is difficult to know which fungicides will be effective here, because our population does not overwinter and is reintroduced from the South each year. Therefore use excellent resistance management practices to avoid allowing the pathogen to develop resistance and to improve the efficacy of your fungicide management program.

Fungicide Resistance management guidelines by crop are available online http://mdvegdisease.umd.edu/Disease%20Management/Fungicide.cfm and hard copies are available in Delaware at the county Extension offices.

Inspect Watermelon and Cantaloupe Transplants, New Bacterial Fruit Blotch Factsheet

Thursday, May 3rd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

As watermelon and cantaloupe transplanting continues on Delmarva, growers are reminded to inspect plants before they are transplanted into the field for signs of disease including Bacterial Fruit Blotch, Gummy Stem Blight, and Angular Leaf Spot.

Kate Everts and Gordon Johnson have put together a new factsheet on Bacterial Fruit Blotch, which will be of interest to watermelon growers. It is available online here: http://agdev.anr.udel.edu/weeklycropupdate/wp-content/uploads/2012/05/BacterialFruitBlotchFactsheet.pdf

Watermelon Pollenizer Variety Selection Matters

Thursday, April 26th, 2012

Emmalea Ernest, Extension Associate-Vegetable Crops; emmalea@udel.edu

For several years now seed companies have been offering special pollenizer varieties for use in the in-row planting scheme for seedless watermelon production. Ideal pollenizers for the in-row system will produce many male flowers throughout the season, be non-competitive with the seedless watermelon plants and possess a high level of disease resistance. Many special pollenizers do not produce marketable fruit.

In 2011 we set up an experiment to evaluate some of the special pollenizer varieties that are available, as well as some seeded Allsweet-type hybrids (large, elongated fruit shape) in an in-row pollenizer planting scheme. The plots in this trial were isolated from one another by at least three rows of seedless watermelons without pollenizers, so we were able to collect meaningful yield data from the seedless melons in the experimental plots and draw conclusions about pollenizer variety performance.

My interpretation of the results from this trial is summarized in the table below. Many of special pollenizers supported a good total yield of seedless melons, however some produced significantly lower yields. In terms of total yield, only one of the Allsweet-type hybrids, ‘Stargazer’, performed as well as the best special pollenizer varieties. The other Allsweet-types were too competitive with the seedless plants, however would still work for dedicated bed systems where they are not growing with the seedless plants. We saw big differences between the special pollenizers in terms of early and late yields. For plantings where you are targeting an early market it will be especially important to choose a pollenizer that isn’t working against your goals. Similarly, pollenizer choice will also affect how long you can keep a field in production, as some pollenizers did not have the longevity of others.

The full report on this trial is available online at: http://ag.udel.edu/extension/vegprogram/documents/2011pollenizer.pdf or by contacting me at (302) 856-7303.

* Allsweet-type hybrid, all others are special pollenizer varieties.

Seedcorn Maggot and Cabbage Maggot Damage Possible in the Next Few Weeks

Thursday, April 26th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

The unusually warm and dry spring we have had up to now has led many growers to transplant some of their melon and other vegetable crops early. The cool wet weather we have had in the last few days will make some of these fields vulnerable to seed corn maggots Delia platura (SCM) or less commonly found in cucurbit fields, cabbage maggots, Delia radicum (CM). Both species overwinter in the soil as a maggot inside a brown case. In March and April small, grayish-brown flies emerge. Adult flies are most active from 10 a.m. to 2 p.m. and are inactive at night, in strong winds and when temperatures are below 50o F or above 80o F. Female cabbage maggot flies seek out and lay eggs on the lower portions of stems of young host seedlings or in nearby cracks in the soil. Within a few days the eggs hatch and the tiny maggots burrow down to the roots and begin feeding. SCM eggs are oviposited in soils with decaying plant material or manure. The adults are also attracted to the organic media around the roots of transplants and germinating seeds. That is why fields that have been fumigated can still have problems with SCM. Maggots will move into small stems and move up the plant causing a swelling of the stem just above ground level, while also causing root collapse and decay. If these stems are split you will usually find the white cylindrical larvae (Photos 1, 2 and 3).

 

Photos 1, 2 and 3. Swollen stem of cucurbit plant with collapsed rotting roots. When stem is cut open the white maggots often can be found.

The adult flies are often found dead, stuck to vegetation during periods of warm wet weather (like we had in early April). These flies have been infected by a fungus, Entomophthora sp. These infected flies usually will be found at the top of a tall object in the field such as a grass seed head or a wire field-flag (Photo 4). Just before the fungus kills them they cement their body via their mouthparts to the tall object and die. If you look closely you’ll see the body is filled with the white fungus that has ruptured between the segments (Photo 5). Being on a tall object allows the spores of the fungus to move longer distances and infect more flies than if the fly had died on the ground. Even though we have had a dry spring, I still have seen many fungus infected dead flies this year. Unfortunately, the infection rate is not enough to reduce the SCM population and stop infestations.

Soil temperatures two inches deep in the planting hole that are at or above 70o F reduce SCM egg laying and larval survival. If soil temperatures are above 70o F at planting but fall below this level for several days in a row (which they have just done), SCM adults will begin to oviposit eggs at the base of transplants. When wilted transplants are inspected in the field, maggots are often not found (they have already pupated), but their tell-tale damage can be seen as a hollowed out stem or root held together by a few strands of plant material. The use of treated seed or in-row banding of an insecticide gives some control of SCM, however, replacing dead transplants is the only solution after SCMs kill a plant. Once seed corn maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative.

Photo 4. Two SCM flies killed by a fungus stuck to a wire field-flag via their mouthparts

Photo 5. Adult SCM killed by a fungus – white strands coming out of abdomen

Understanding Seed Waivers and Seed Born Diseases

Thursday, April 26th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A number of vegetable diseases can be transmitted through infected seed. For this reason, seed companies have developed quality assurance programs based on testing a certain amount of seeds in each seed lot for specific diseases. This most often involves the seed company growing out plants from these test lots and having trained individuals inspect the plants for signs of the disease. If there are suspicious plants, they are then further tested in the laboratory to confirm the disease. Sometimes seeds are tested directly for the specific disease organism (bacteria, virus, fungus). Only seed lots that have no disease detected are sold. For watermelon and cantaloupes, seed lots are tested for bacterial fruit blotch and often for gummy stem blight.

Because of past liability issues, growers are required to sign waiver forms to purchase watermelon and cantaloupe seeds from most companies. While this is often thought by growers to be a routine annoyance to purchase seeds, it is important to read the waiver forms and understand their implications.

These waivers commonly spell out what diseases the company tests for. The waiver will often have information on the testing process for these diseases. There will also be information about the diseases that the grower should know and often there will be detailed descriptions of how the disease develops and how to identify the disease.

In all waivers, there will be an important statement emphasizing that that the grower accepts the risks associated with those diseases.

The waiver may also include information on risk of nonperformance, assumption of risk, disclaimers or limitation of warranties, limits of liability, limits on damages, how to file a claim, statute of limitations on claims, arbitration of seed disputes (required by some states), expected remedies, limit on sales or transfers of seed, and attorney’s fees.

Once a seed waiver is signed then the seed company is protected from liability and this will reduce the ability of a grower to receive compensation if a seed borne disease does appear.

All growers are encouraged to understand what seed borne diseases are common with the vegetable crops that they grow, whether or not seed is treated or tested to reduce the chance of disease occurring, how to identify specific seed borne diseases, and how to manage seed borne diseases if they do occur (in greenhouse transplants or field plantings).

Growers should also maintain close relationships with seed suppliers and contact them immediately if a seed borne disease is suspected.

Updated Fertilization Recommendations for Drip Irrigated Crops

Thursday, April 19th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Extension specialists in the Mid-Atlantic have updated fertilizer recommendations for drip irrigated plasticulture production of crops in the Commercial Vegetable Production Recommendations. The following are recommendations for watermelons and tomatoes.

Suggested Fertilizer Program Using Trickle Irrigation for Watermelons

Days After Planting

Daily

Cumulative

Nitrogen1

Potash1,2

Nitrogen1

Potash1,2

——————–lbs/A——————–

Preplant3

25

50

0-14

1.0

1.0

39

64

15-28

1.5

1.5

60

85

29-56

2.0

2.0

116

141

57-78

1.5

1.5

137

166

79-93

1.0

1.0

150

175

1Adjust rates accordingly if you apply more or less preplant nitrogen and potash.
2Base overall application rate on soil test recommendations.
3Applied under plastic mulch to effective bed area using modified broadcast method. Adjust as needed.
Note: recommendations are based on 8 foot bed centers. If beds are narrower, fertilizer rates per acre should be adjusted proportionally. Drive rows should not be used in acreage calculations.

Suggested Fertigation Schedule – Fresh Market Tomatoes

Days After Planting

Daily

Cumulative

Nitrogen1

Potash1,2

Nitrogen1

Potash1,2

——————–lbs/A——————–

Preplant3

50

125

0-14

0.5

0.5

57

132

15-28

0.7

0.7

67

142

29-42

1.0

1.0

81

156

43-56

1.5

1.5

102

177

57-77

2.2

2.2

148

223

78-98

2.5

2.5

201

276

1Adjust rates accordingly if you apply more or less preplant nitrogen and potash.
2Base overall application rate on soil test recommendations.
3Applied under plastic mulch to effective bed area using modified broadcast method. Adjust as needed.
Note: recommendations are based on 6 foot bed centers. If beds are narrower, fertilizer rates per acre should be adjusted proportionally. Drive rows should not be used in acreage calculations.

Additional recommendations can be found in the Recommendation which is also online at this site: http://ag.udel.edu/extension/vegprogram/publications.htm.