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Compost, rhizobacteria and row covers: a way to manage nutrients and pests in organic cucurbit production

Posted: February 23, 2010

Organic cucurbit growers face two main challenges, managing insect pests and using organic nutrient sources. Researchers at Penn State are investigating innovative strategies to manage cucurbit problems more sustainably through a multidisciplinary approach that integrates key components of pest management and soil fertility. The goal is to optimize promising strategies which apply to organic cucurbit production throughout the eastern United States.
The muskmelon trial in 2009. Plants were covered until flowering, covered until 10 days after flowering or not covered (starting from second row on the left in that order, respectively).

The muskmelon trial in 2009. Plants were covered until flowering, covered until 10 days after flowering or not covered (starting from second row on the left in that order, respectively).

Cucurbits such as cucumber, melons, squash, pumpkins, etc., are known to have two severe pest problems in the eastern region of the U.S. The striped cucumber beetle (Acalymma vittatum), which transmits the bacterium Erwinia tracheiphila causing bacterial wilt disease and the squash bug (Anasa tristis), which transmits Serratia marcescen the causal agent of cucurbit yellow vine disease (CYVD). On the other hand, organic nutrients sources such as compost, manure or plant residues, release nitrogen slowly as organic nitrogen is converted to inorganic nitrogen through the process of mineralization.

During 2006 a group of researchers from Iowa State University (ISU) started investigating the effect of row covers on bacterial wilt disease and yield in muskmelons. This group took an innovative approach to using row covers to exclude insects by extending the row-covered period after plant flowering. Their preliminary findings suggested that extending the row-covered period beyond the start of bloom could improve protection against bacterial wilt without sacrificing yield or earliness. However, the method of using the row cover only protected plants for a period of time since it must be removed in order for the crop to be pollinated by bees.

That led our Penn State group to conduct a two year experiment at the Russell E. Larson Agricultural Research Center near Rock Springs, PA to evaluate the effect of using various plant inducers as seed treatments then growing cucumber transplants under row covers. One of the plant inducers was plant growth promoting rhizobacteria (PGPRs), which can activate natural plant defense mechanisms. Our findings in this experiment were that PGPR significantly reduced bacterial wilt severity and delayed incidence until the end of the season. With all these findings both teams at PSU and ISU along with researchers from the University of Kentucky proposed to develop practical management strategies for pests and nutrients that can be adopted immediately by organic cucurbit growers throughout the eastern half of the U.S.

cucurbit graph
Bacterial wilt disease severity during the 2009 growing season in muskmelon. Lower severity of the disease was observed in row covered treatments removed 10 days after flowering.

During summer 2009 the first study was established at the Russell E. Larson Agricultural Research Center in PA. We investigated the use of row covers on a muskmelon crop by removing them at flowering or removing them 10 days after flowering started. We assessed striped cucumber beetle populations and bacterial wilt severity during the growing season in the muskmelon crop. Our preliminary data showed that striped cucumber beetles and therefore bacterial wilt were significantly suppressed, by 31%, with the use of row covers removed at flowering and by 44% by removing row covers 10 days after the start of flowering compared to not using row covers (control) (Fig 1).

During this same trial we also evaluated the use of dairy and food scrap based compost assuming a 10% or 30% mineralization rate and organic bagged fertilizer (control).  Transplants were produced with or without PGPR (Bacillus subtilis) as a seed treatment. We found various responses of the different combination of PGPR and compost treatments for each row cover treatment. However, it is interesting to note how a low application of compost (assuming a 30% mineralization) with the use of PGPR had the lowest bacterial wilt pressure in muskmelon.

We had learned from previous research that various compost types can benefit disease management by increasing activity and diversity of soil microflora. However, it is difficult to know from this study what targets the plant to be resistant to the disease. Our next question addressed was: what effect do row covers combined with dairy and food scrap based compost and PGPR have on yield? As expected, increased yield was observed by the use of row covers removed at flowering (18%) and row covers removed10 days after flowering (37%) compared to not using row covers (control). Surprisingly, no significant increase was observed in yield among each combination of compost and PGPR’s integrated with the use of row covers. This is good news for growers since lower rates of compost had a similar effect to higher rates on plant yield.

We also evaluated the use of row covers, different rates of compost and the use of PGPRs on butternut squash.  In this trial we added another treatment which consisted of using row covers during the entire season integrated with a bumble bee hive. Preliminary results showed differences in yield response compared to the muskmelon trial. It was interesting to see that yield was increased by 10% when row covers were removed at flowering compare to not using row covers. However, removing row covers 10 days after flowering or leaving them on all season reduced yield by 34% or 42%, respectively.  This disparity between muskmelon and butternut squash may be due to various factors. Squash bug is one of the main concerns for winter squash, however, during this trial year populations of this pest were very low. Therefore, plants were not under stress due to this pest.  Also, high temperatures after flowering may have triggered plant hormones increasing vegetative growth and reducing flower development. We will need to evaluate these responses under greenhouse settings to further understand what may be occurring.

By Ermita Hernandez and Elsa Sanchez, Penn State