An Analysis of Cucurbitaceous Leaf Wilt

By J.D. Adams

 

Are your garden plants withering and dying, growing slowly or dwarfing? Members of the Cucurbitaceous family, including pumpkins, cucumbers and zucchini are susceptible to the leaf wilt caused by the bacterium Erwinia tracheiphila, but it can also spread to tomatoes, potatoes, and certain other vegetables. The withering blight has multiplied significantly since 2000 along with the striped and spotted cucumber beetles (Acalymma trivittatum and Diabrotica undecimpunctata) that spread it. Infection by the bacterium Erwinia tracheiphila can ruin the entire crop yield of an area; consequently the potential global impact of the problem is enormous. Another soil-dwelling creature that can impact the health of your garden is an arthropod of the class Symphyla, resembling a white centipede about ¼ inch in length. These are fast-moving herbivores that feed on the roots of sprouts and young plants, being capable of fast movement throughout the soil.

 

 

Pumpkin Flower with Beetle-1.jpg


Erwinia tracheiphila cannot withstand cold temperatures, but the exact mechanisms by which it survives the winter have yet to be fully reconstructed. Research into the theory that the primary method of survival is overwintering in the digestive systems of cucumber beetles has yielded conflicting data, and more scrutiny should be directed at overwintering environments created with heat generated by composting material. Whether the beetles emerge in the spring already infected to feed on young garden plants, or whether they become infected by early feeding on diseased plants or by some other means is an evolving issue for the mitigation of leaf wilt.

With warm temperatures in spring, you may notice signs of cucumber beetle larva infestation in young plants, which normally grow rapidly and develop leaves several inches across. The two pumpkin plants below exhibit signs of dwarfing causing by larva feeding on the root system. While seemingly healthy in some ways, the leaves are miniature and the yellowing is a sign of poor absorption of nutrients. Leaves without chlorophyll cannot manufacture energy for growth. Examination of the rootball and planting site will reveal the cucumber beetle larva, which are about ¼ of an inch long and white in color. The rootball of each plant can be submerged in water for 2-3 hours to eradicate the larva. If you can hold the rootball together during this process the plants will recover, otherwise you’ll find pumpkins and zucchini are significantly delayed by transplanting shock. The best method to remove plants from the ground is to use two trowels, one on each side to hold and lift the rootball intact. If the plants are small they can be transferred to peat pots immediately after being removed from the ground, and then placed in the water to submerge the roots for 2-3 hours. The technique is scaled up for larger plants that can be isolated from infected ground with container plantings that conserve heat, described later in the article. In the U.S., managing impact of the garden Symphylans (Scutigerella immaculata) on crop production is focused on detection and types of crop rotation, as the arthropods tend to form hotspots in the soil from year to year plantings in the same location. Aside from greenhouses and rotating the locations of plants, container plantings are one of the few organic methods that can isolate the plant from both Symphyla and cucumber beetle larva.

 

 

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The affected soil in the planting site can be sterilized by spreading it out in full sun on black plastic. The high temperatures and dry soil are inhospitable for the larva. Insert a shovel in the old planting site and twist it around several times to control any remaining larva. Replant the pumpkins as soon as possible in a location removed from the original site by some distance, and water immediately. Plants in peat pots will be isolated from the ground long enough that the plant can recover without being attacked by larva, which feed on the fine root hairs.

 

 

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As the spring progresses cucumber beetle residue infected with Erwinia tracheiphila bacteria are introduced into damaged leaves, flowers, and stems. The bacteria reproduce in the vascular system, increasing the viscosity of the transported water until the system fails. In a typical scenario first the leaves wilt, associated stems and vines will collapse, and eventually the entire plant dies. The cucumber beetles and bacteria are closely related such that controlling the beetles will limit the scope of infection by the bacteria.

 

 

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My garden became a botanical laboratory for the purpose of this research. The bacterial wilt was studied on 3 varieties of giant pumpkins of the species Cucurbita maxima, “Big Max”, “Big Moon”, and the “Weeks NC Giant”. The seeds were sprouted indoors in May and planted in June after the first wave of cucumber beetles had passed. Individual plants were spaced 10 feet apart so they could be studied in relative isolation to each other.

 

 

Weeks NC Pumpkin-1.jpg


Standard techniques for organic control of leaf wilt have evolved over the past several years, as follows:

Grow varieties that tolerate bacterial wilt like butternut or acorn squash, or the County Fair Hybrid cucumbers.

Remove diseased plant material when wilting is first noticed. Cucumber beetles can spread the bacterium by traveling around or by feeding.

Row covers can deter cucumber beetles from feeding and infecting plants by covering the plants with netting.

Avoid close planting of cucurbits and corn as spotted cucumber larvae also feed on corn.

Grow susceptible crops on rotation every third year since beetles overwinter in plant residue or the soil and carry the bacterium. The cycle can be broken by only planting the crop in an area every third year.

Rototill in the fall to expose any cucumber beetle eggs to the elements.

Water only around the base of each plant, keeping the leaves as dry as possible.

Water deeply every few days, morning or early afternoon.

Plant later in the spring after the cucumber beetles emerge from their hibernation.

Use yellow sticky cup traps to catch roaming cucumber beetles.

Hand-picking beetles on smaller gardens is effective.

Foliar application of kaolin clay with a spray bottle deters cucumber beetles.

By observation of the progression of Erwinia tacheiphila in leaf structures I’ve developed new organic techniques for mitigating the progression of the disease with an understanding of the structural peculiarities of the Cucurbitaceous family (cucurbits) that make it susceptible to this bacterial blight. Limited control is possible once infection has occurred, as I will soon explain.

As of 2015, the prevailing theory is that most or all of the recent proliferation of the bacterium Erwinia tracheiphila is attributed to the cucumber beetles, due in part to shifts in North American climate. While noteworthy, my research shows it is complicated by vulnerabilities in the leaf structure of cucurbits. Also the involvement of other insects in overwintering and transmission needs more review, and several aspects of the progression of Erwinia tracheiphila through the plant should be noted.

The fact that the Cucurbitaceous family is one of a small number of plants that possess the so-called water stomata or hydathodes has been an overlooked aspect of bacterial transmission in the case of leaf wilt. These structures are typically present along the serrated leaf edges of pumpkins and other cucurbits. The development of serrations is an adaption to wetter climates; it’s theorized that the serrations facilitate the formation of water droplets to help drain water from the leaf surface, while also improving the dynamics of respiration. Transpiration is performed by small pores on the surface of leaves called stomata that allow molecules of water to evaporate while blocking the entry of bacteria.

Hydathodes are larger pores along the edges of the leaf that are involved in a process called guttation, which is the exudation of drops of xylem sap on the edges of leaves of some vascular plants such as pumpkins. It shouldn’t be confused with dew, which condenses from the air onto plant surfaces. Guttation occurs during periods of high humidity and/or rainfall when excess water is drawn in by the roots, moving upward through the plant and exiting though the hydathodes. These openings are large enough to allow water in liquid form to flow out, and therefore are also large enough for the bacterium Erwinia tracheiphila to enter.

Presumably the traveling cucumber beetles have spread the bacterium over the surfaces of leaves, flowers, and stems. Dew or rainfall can then wash the bacterium into the hydathodes where it will co-mingle with the moisture present, eventually spreading to other parts of the plant. The progression of Erwinia tracheiphila is seemingly relentless, yet one that could respond to proactive antiseptic techniques. At the following link the hydathode mode of bacterial transmission in plants is detailed:

http://bugs.bio.usyd.edu.au/learning/resources/PlantPathology/infection/image_pages/hydathode.htm

Typically the oldest leaves on pumpkins plants are the first to fall victim to the leaf wilt. Conventional wisdom might assume that beetle feeding injuries would occur over the entire length of the plant, and so damage and infection would be spread out over the entire plant. Reports suggest however that beetles prefer the affected plants over the healthy plants, perhaps because infected cucurbits exhibit increased branching and flowering. This behavior suggests that any breakdown in the plants defenses gives rise to an exponential increase in exposure through the hydathodes and tissue damaged by beetles, feeding back into the cycle with exposed necrotic plant tissues. Erwinia tracheiphila can also enter the plant through damaged flowers, which are favored by the cucumber beetles. The role of honeybees in spreading the bacterium as in the case of the Fire Blight Erwinia amylovora should be quantified, along with other insects that visit cucurbit flowers.

 

 

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The younger leaves have a degree of resistance from infection, especially in the case of pumpkins and the gourd-like species Cucurbita maxima. The oldest leaves have endured the feeding damage for a longer period of time, sustaining major intrusions of Erwinia tracheiphila, but the prevalent pattern of leaf browning and necrosis indicates that condensation on the tip or lowest edge of the pumpkin leaf is a major factor in the impending failure of the entire leaf and eventually of the plant itself.

 

 

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In my research the overall health of the giant pumpkins was dominated by the degree of infection, hence the condition of individual plants varied widely. The variety of pumpkin did not correlate to any identifiable resistance to the leaf wilt. Other factors impacting the health of the test pumpkins included the physical placement in the garden area, the proximity to planted corn and infected plants, and the source of compost used. Cucumbers and zucchini have less natural immunity to be leveraged, but did respond to basic organic techniques for insect control and bacterial mitigation.

The list of forces driving the infestation therefore should include:

1)    Cumulative exposure to the bacterium through the hydathodes.

2)    Movement and feeding of infected cucumber beetles.

3)    Proximity to diseased plant residue.

4)    Temperature, moisture, and climate. 

 

 

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Pruning of affected leaves from the vine can effectively delay the spread of Erwinia tracheiphila to the rest of the plant. Leaves that have significant wilting or browning should be trimmed off, along with leaves where the affected area approaches the stalk (petiole). Diligent monitoring of the condition is required for this to be effective. The overlapping leaf pattern of pumpkins creates a domino effect of bacterial transmission from the base to the tip of the plant if left unchecked. On small to medium size fields, pruning to mitigate leaf wilt can be done to break the pattern of transmission. Looking upward from the base of the plant, note the last leaf on each side of the main vine which has noticeable wilt, yellowing, or necrosis and remove it with a scissors that has been sterilized briefly with isopropyl rubbing alcohol. Cut the stalk at the upper end where it attaches to the leaf. The pruning deters bacterial infection by blocking direct contact with adjacent leaves, creating an obstacle to beetle movement, and limiting internal transmission to the rest of the plant.

Isopropyl alcohol of 50% aqueous solution can be practical to use on smaller fields of cucurbits to treat affected leaves that display yellowing or wilting along the outer edges. Alcohol kills bacteria and limits leaf burn by evaporating quickly. The alcohol can be applied with a spray bottle to control Erwinia tracheiphila that has been introduced onto the leaf surfaces and into the hydathodes through guttation. The most effective time of application may be during or just prior to periods of guttation to limit bacteria from entering the vascular system of the plant. Affected leaves that are salvageable can be sprayed with alcohol that will flow into the hydathodes before evaporating. Concentrate the application on the lowest leaf edges as most condensation and any bacteria present will flow past this point. Low-hanging leaves, if infected, should have the ground under the leaf sprayed with alcohol. Frequent applications of isopropyl alcohol will burn the leaves unless further diluted with water. Continuing research should investigate the use of methanol alcohol as a disinfectant; many plants show increased growth after applications of methanol alcohol.

If cucumber beetle numbers are controlled with long-term management, leaf wilt can be successfully minimized and crop failure avoided. Some people use a portable vacuum to eradicate the beetles; others use a thin wand with sticky tape or adhesive to pick off the bugs. A perfect implement is a used grill-lighting match like the 11 in. ones made by Diamond, with a dab of corn syrup on the flat end of the matchstick. At the start of this experiment while beetle numbers were still moderately high, I observed increasing numbers of predator insect species such as harvestman spiders and ground beetles. When cucumber beetle numbers dropped off due to organic eradication, these natural predators were able to maintain low numbers from that point forward. The best plant specimen in the garden was adjacent to plantings of catnip and broccoli, both of which are repellent to cucumber beetles.

In cases where the previous year had seen significant numbers of cucumber beetles, it may be necessary to go fallow for a year to decrease the infestation. It’s possible to raise plantings that are isolated from the ground with containers that rest on a pad that brings heat from the ground upward. In the photo below I’ve used two layers of bricks to transfer the Earth’s heat into a container planting. In the spring and early summer the temperature of the ground is gradually rising toward 60 degrees F. The problem with containers is that in the spring the cool nights will delay the sprouting of seeds because the plant will assume that if the temperature is getting down to 40 degrees it must be too early to sprout. In the ground that doesn’t happen because its temperature is much more stable.

 

 

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The combination of improved transfer of ground heat and the solar radiation on the sides of the dark containers warms the roots and increases growth above that of an ordinary planting. Pumpkin vines will tumble over the side of the container and root themselves as they travel across the ground. By that time the plants are well established and can resist infestation. Seeds planted in these containers sprout within a week with a near 100% success rate.   

Robust plant growth offsets the debilitating effects of Erwinia tracheiphila. In well-worked, well-drained soil cucurbits can resist leaf wilt for an extended period of time. Controlling the numbers of Symphyla with tilling and crop rotation will minimize root damage and ensure rapid growth. The specific conditions necessary for growing pumpkins require that soil temperatures three inches deep are at least 60 °F (15.5 °C).

CONCLUSION: Out of 24 test pumpkin plants and in the presence of the invasive bacterium Erwinia tracheiphila, after four months of growth I had all 24 plants surviving, 8 plants that were healthy, producing pumpkins and relatively pristine, 12 that had from 10% to 30% of the wilted leaves trimmed off but were also otherwise healthy, and 4 that exhibited stunted growth but were blooming and showing no sign of imminent demise. I attribute this success in the face of a grim prognosis to organic insect control, defensive pruning, and to the targeted application of an anti-bacterial, environmentally benign, non-residual treatment to Cucurbitaceous plant surfaces at risk from the pathogen Erwinia tracheiphila.

Copyright October 16, 2015 by J.D. Adams

Email the author

Supporting information:

http://www.majordifferences.com/2014/12/difference-between-stomata-and.html#.Vh9e530oGB1

http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1037&context=pss_etds

http://droyer.web.wesleyan.edu/leaf_teeth%28IJPS%29.pdf

http://plantcellbiology.masters.grkraj.org/html/Plant_Cellular_Physiology7-Loss_Of_Water_I-Guttation.htm

http://www.nytimes.com/1992/11/03/news/increasing-plants-yields-with-sun-and-diluted-methanol.html

http://library.uark.edu/search/o9598954

http://ento.psu.edu/directory/sjf4/seroEstSeasonalDynam.pdf

 

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