Three years of trials were implemented on a commercial watermelon field leased from a local grower and greenhouse/laboratory experiments conducted to determine the biology of and management techniques for Mature Watermelon Vine Decline (MWVD). It was determined in greenhouse experiments that a biological factor in the soil is responsible for MWVD. The onset and severity of MWVD appears to be correlated with excess soil moisture. Canola used as a cover crop may mitigate the severity of MWVD. Although biological inoculants used in the early season increased the vigor of watermelon plants, the severity of MWVD was not reduced.
Since the mid-1980s, watermelon fields in southwestern Indiana have experienced outbreaks of a disease syndrome known as Mature Watermelon Vine Decline (MWVD). The disease was a limiting factor in production and reduced yields in many fields in 1989, 1995, and 1999. In 2000, it was especially severe, affecting more than 50 percent of watermelon acreage in southwest Indiana, and reducing total estimated yield by 20 percent. (Egel et al., Plant Health Progress, 2000 doi:10.1094/PHP-2000-1227-01-HN).
Initial MWVD symptoms include necrosis and wilting of leaves, followed by the wilt and collapse of the vines. Vine collapse reduces fruit quantity, size, and quality; prevents normal ripening; and exposes fruit to sunburn. On symptomatic plants, the root systems are generally sparse — the primary roots are necrotic (dead tissue) and the plant has few secondary roots.
Symptoms often appear on mature plants in low, poorly drained areas. Under the right conditions, MWVD incidence will increase through the summer, often resulting in the collapse and decline of large portions of affected fields. Plants with MWVD often yield no marketable fruit.
Management of MWVD is dependent on knowing whether the cause is biological in nature or the result of environmental factors and compromised root systems. More information about how and why MWVD occurs will assist management.
Cover crops in the Brassica family release a biofumigant (thiocyanate) upon degradation in the soil. For this reason, Brassicas have been used for management of soil borne diseases (Managing Cover Crops Profitably, 3rd edition, SARE). Experiments conducted here were designed to determine whether watermelon benefit from using canola as a cover crop to reduce the severity of MWVD.
Biological inoculants are products containing live microorganisms and are used for pest management and/or to increase nutrient up-take and other benefits. Growers have long wondered if such products have the capacity to add back to the soil some of those properties that human cultivation has taken away. Several such products were used in these trials for possible management of MWVD.
- Determine whether the cause of MWVD is biological in nature.
Determine the efficacy of cover crops and biological inoculants in the management of MWVD.
Communicate the results to stakeholders
Soil from a commercial watermelon field with a history of MWVD was placed in 5-gallon pots and either fumigated or left unfumigated. Soil was fumigated with 5 grams per pot of Basamid (active ingredient, dazomet) by mixing the fumigant in the soil to a depth of 6 inches after which the soil was watered thoroughly. Four weeks later, 3 tablespoons of Osmocote (19-6-12) was added to each pot and then each pot was planted to either watermelon (Royal Sweet) or muskmelon (Eclipse) in a greenhouse setting. The plants were watered as needed until 37 days after planting when each pot was watered 3 times per day with 1 liter of water for 7 days to induce MWVD. Plant health and incidence/severity of MWVD were observed daily. Experimental design was a completely randomized design with 6 replication.
Field plots were planted in a commercial watermelon field that had a history of MWVD. In the fall of each year, plots were planted to either canola, winter rye or left in bare ground. Canola variety ‘Sumner’ was planted at 8 lb/A and winter wheat of a local blend was planted 60 lb/A. Cover crops were planted 9 Oct in 2006, 17 Sep in 2007 and 16 Sep in 2008. Each spring, the watermelon hybrid Royal Sweet was planted at 3.5 in-row spacing on 6-foot centers in 50-foot rows and either treated with the biological inoculants BioYield, T22 or left untreated (control plants were treated with water) when the plants were just beginning to vine. In the third year of experiments, Bioyield was unavailable and the biological inoculant Actinovate was used instead. Bioyield is a mixture of bacteria (Paenobacillus macerans and Bacillus amyloliqueaciens) and was used at a rate of 40:1 (v:v) in the transplant mix; the active ingredient of T22 is Trichoderma harzianum Rifai strain KRL-AG2 and was used at the rate of 2 oz/A; the active ingredient of Actinovate is Streptomyces lydicus and was used at the rate of 12 oz/A.
Data collected included vine growth, nutrient analysis of leaf tissue, disease severity and yield.
The experimental design was a randomized split plot with cover crop the main plot. There were 4 replications.
Only watermelon planted in soil that was planted in non-fumigated soil was affected by MWVD in greenhouse experiments. This means that fumigation destroyed a biological component responsible for MWVD. Muskmelon plants were unaffected by MWVD, an observation supported by field observations. The onset of MWVD in these pot experiments followed treatment of all pots with large amounts of water. This observation from the pot experiments also supports observations from the field.
Canola planted in October, as was done in the first year of the project, did not overwinter well and produced little biomass to be cultivated in the next spring (field season 2007). When the canola was planted in mid-September, survival was good and there was plenty of biomass the next spring (field season 2008, 2009). This planting date would work well after a vegetable crop, e.g., watermelon. Most agronomic crops, such as corn or soybeans would not be harvested by mid-September.
Vine vigor was increased by the bio-inoculant Bioyield as compared to the untreated control in 2007 and 2008. For example, watermelon vines that had been treated with Bioyield as transplants had greater vine length and number of nodes compared to the untreated controls in 2007. In 2008, watermelon plants in bare ground plots (no cover crop) treated with Bioyield outyielded plants left untreated in bare ground plots by more than double. There was some evidence that Bioyield and T22 should not be used together.
In 2007 and 2008, watermelons that were planted in canola plots had several micronutrient levels that were higher than in the winter rye or bare ground treatment. However, only zinc levels were consistently higher in the canola treatment in both 2007 and 2008. In 2009, no differences in micronutrient levels were observed in any treatment. Cover crops had no consistent affect on watermelon yields. Biological inoculants did not affect micronutrient levels.
On 2007, MWVD was observed in the experimental plots. This was the only occasion that MWVD was observed in the plots. Watermelon in canola plots had significantly less vine collapse as a result of MWVD than the winter rye treatment. The amount of MWVD in canola treatments was also less than in the bare ground treatment, but not statistically so.
In 2006, just as this project was starting, a field of watermelon with vine decline symptoms was observed in southwest Indiana. The virus squash vein yellowing virus was confirmed from this field. It was the first report of this virus outside of Florida. Squash vein yellowing virus is transmitted by the whitefly and is known to cause a vine decline of watermelon. It is unlikely that numbers of whiteflies in Indiana could become significant enough for this virus to be an annual disease problem for Indiana growers.
A grower meeting to discuss the outcome of this project as well as to survey the growers about this project was poorly attended. Therefore, the survey was not presented to the growers. As another method of assessing the impact of the project, the numbers of instances that MWVD has been diagnosed in Indiana in the last 11 years was enumerated. According to diagnostic records of the Southwest Purdue Agricultural Center, the instances of MWVD are as follows: 1999, 15, cases; 2000, 8, cases; 2002, 7 cases; 2003, 2 cases; 2004, 0 cases; 2005, 5 cases; 2006, 3 cases; 2007, 4 cases; 2008, 3 cases; 2009, 0 cases and 2010, 0 cases. These records reflect weather related patterns as well as the incidence of MWVD. Overall, however, the number of MWVD related diagnoses have gone down. Years with much rain during the season often have more MWVD reported. However, 2009 and 2010 have been very wet years and no MWVD was reported. In addition, the number of calls from growers to report MWVD has decreased.
In 2000, it was estimated that 20% of the watermelon yield was lost due to MWVD. Watermelon yield that year was 260 cwt/A and total revenues for that year were $10,150,000. Yield and revenue have risen steadily since that date: In 2006, the latest figures available, yield per acre was 370 cwt/A and revenue was $26,011,000. Certainly a portion of the health of the watermelon industry in Indiana is due to the reduced presence of MWVD.
Although the survey was not presented to growers due to low attendance at the meeting mentioned above, the statistics regarding the reduction in MWVD incidence during the last several years indicates that watermelon growers have adopted the Purdue University recommendations for MWVD management that grew out of this project:
• To avoid the potential buildup of disease-causing organisms in the soil, it is recommended to rotate crops away from watermelon and other cucurbits in areas where MWVD is a problem.
• Watermelon fields should be irrigated judiciously and fields with poor drainage avoided.
• Some types of cover crops (such as canola, winter rape and mustards) may help reduce the severity of MWVD.
• Our research results, as well as those of others, show that plants with well-developed root systems resist MWVD better than those with poorly developed root systems. Therefore, cultural conditions that promote good root development should be practiced.
More details about these recommendations can be found in the “Mature Watermelon Vine Decline and Similar Diseases of Cucurbits (BP-65)” cited above.
Educational & Outreach Activities
Two outreach meeting were held about this project. In February 2009, at the technical meeting of the Southwest Melon and Vegetable Growers Association, data from the two years of experiments was presented: “The Use of Cover Crops to Manage Soilborne Diseases of Watermelon”- 77 growers attended. Several questions were asked both during and immediately after the presentation as well as individuals who asked questions privately.
SARE Grant Field Day – There was a field tour of the experimental plot as well as a discussion of cover crops and biological inoculants – Southwest Purdue Agriculture Program – 29 Jul 2009- 25 people attended.
Egel, D.S. and S. Adkins. 2007. Squash Vein Yellowing Virus Identified in Watermelon in Indiana. Plant Disease 91(8) 1056 DOI: 10.1094/PDIS-91-8-1056B.
Egel D.S. and Martyn, R, 2010. Mature Watermelon Vine Decline and Similar Diseases of Cucurbits. Purdue University, BP-65-W <http://www.extension.purdue.edu/extmedia/BP/BP-65-W.pdf>
Areas needing additional study
Areas Needing Additional Study
The use of a flail mower to chop the canola into smaller pieces immediately before incorporating the residue might extract more bio-fumigant and thus enhance disease management. This could be the subject of another experiment or on-farm trial. The bio-inoculant Bioyield increased vine growth more than the other bio-inoculant treatments. However, Bioyield was added at seeding while the other products, T22 and Actinovate, were added at first vine. The subject of another experiment might be to add all bio-inoculants at seeding or early transplanting. Additional work could add bio-inoculants several times during the season through trickle irrigation. In this way, it might be possible to produce increased growth due to bio-inoculants the entire season, not just the initial portion.