By David F. Graper
SDSU Extension Horticulturalist
Gardens are eco-systems unto themselves with lots of various components from the microorganisms in the soil, worms, insects, weeds, and some animals plus the vegetables that we plant and hope to harvest. As much as we might like to think we are the chief of our garden and should get to eat whatever we want, there are other critters out there that want their own “piece of the pie” or in this case their own share of the sweet corn.
Most gardeners are pretty generous with their excess produce – who hasn’t been the recipient of someone’s extra zucchini from time to time? The problem with raccoons though is that they are pretty greedy. Not only that, by the time a few of these nighttime foragers get done, there is often very little edible sweet corn left to harvest. They tend to knock lots of it down, rip open ears, take a few bites, shred the rest of the cob a bit then move on to the next stalk. It can be a pretty discouraging site after you have been tending your sweet corn patch all summer long, and now, waiting with mouth-watering anticipation find that the day or two before you were going to start harvesting that the ‘coons got to the corn first.
So what is a gardener to do? If you ask around, or check online you can find lots of different suggestions. In my experience, the best option is an electric fence around the perimeter of the sweet corn patch. If you have a building with electricity close to your garden, you can purchase an inexpensive fencer starting at about $60. If that is not an option, a solar powered fencer can be installed just about anywhere. Those units can be picked up starting at about $150. Keep in mind you can buy a lot of sweet corn for $150 but for many, the taste of freshly harvested sweet corn from your own garden is worth it.
If you are not familiar with how electric fencers work, be sure to read the instructions. You will also need the wire or fencing tape, posts, insulating buttons of clips and a grounding rod. I use 6’ T-posts and fencing tape. It is wider than regular fencing wire, somewhat easier to work with for a small installation like this but is more expensive. It is also easier to see so you are less likely to run into it and get shocked and it should be more visible to the critters we are trying to keep out as well. I install four layers of wire. The first one is about 6” above the ground with a second one 6” higher. I then add one more in the middle of the post followed by a final one at the very top. The bottom two are for the ‘coons while the top two help to deter deer that also love to eat sweet corn and knock it down. You should also have a way to easily turn off the fencer to allow you easy access to the sweet corn, or you can add a fence gate with an insulated handle that you clip onto the fence to close it or unhook it to open it. The solar powered fencer that I have has a simple switch right on the front so I can turn it off. Believe me, grabbing on to or bumping into the wire while it is on is a mistake that you will likely try to avoid repeating. Just remember to turn the fence back on after you are done or the ‘coons will likely find a free meal.
There are a few other things to keep in mind or check before you purchase your fencer. Some of the solar powered fencers reduce the number of pulses that they put out during the night to conserve battery power. That is probably fine for cattle that usually sleep at night but not the best option when you are trying to keep things out at night. Fencers put out periodic pulses of electricity, probably about once per second. Most fencers will have a light that blinks with each pulse. You can check if your fencer is working by using a fencer tester or by manually shorting out the wire to one of the posts with something metal that also has an insulated handle, like a screwdriver. Hold it on the wire and close to the post without touching it and watch for the spark and snapping sound it makes. If it does not appear to be working, walk around your fence and listen for that snapping sound where a short might have developed. Look for weeds growing up into the wire or corn stalks or leaves that are leaning down onto the wires, which will also cause a partial short of the fence, cutting down on the jolt that will be delivered when an animal or you touch the fence. At the end of the sweet corn season you can wind up your wire or tape, pull out the posts and save them for next year.
Live or box traps can also be used to assist in dealing with raccoons and skunks, which may also get into sweet corn. These can be picked up for about $30. The key is adding a bait that will be very attractive to the raccoons and skunks and not trap dogs or cats. Sweet food items often work best. Try marshmallows, honey, sweetened cereals or hard candy. Eggs and sweet corn cobs can also be used. Fish and cat food are also excellent baits but you will likely catch cats before raccoons. Raccoons can be very smart and even escape traps. I have had them pull weeds from between the wires on the bars, under the bottom of the door to make a crack large enough to get themselves out. So, be sure to check your trap right away in the morning and again during the day to make sure you don’t have a cat or dog trapped without food or water. Once you have captured a raccoon or skunk you will need to decide how you are going to remove it from your garden. People will often try to transport the animal, inside the trap out to the country somewhere to release it. Be very careful to avoid direct contact with the animal to avoid being bitten, scratched or, in the case of the skunk, getting sprayed. Skunks in particular often have rabies, so be careful! Trapping and releasing might be a good option but the animal might come back or end up getting into someone else’s garden. You might consider other options or sometimes your local animal control can help too.
Mexican Bean Leaf Beetles
One of the more interesting vegetable garden pests you may encounter is the Mexican bean leaf beetle (Epilachna varivestis). While the adults are not all that exciting, the larval stages are quite striking as they are yellow-orange and covered in black-tipped yellow spines as they grow larger. The larvae tend to do most of the damage to bean plants since they feed for a longer time as they progress through several generations before pupating and emerging as a copper-colored adult that looks similar to but larger than a lady beetle. The adults have 16 black spots on their wing covers. There are one to three generations of this beetle per year. The adults lay clusters of bright yellow eggs on the undersides of the leaves. Once the eggs hatch, there are four larval instars per lifecycle. Damage from these insects is called skeletonizing because the earlier instars only eat part-way through the leaf leaving the veins behind. Later instars can eat all the way through the leaves but they make scattered holes in the leaves. The adults may also feed on flowers and pods.
There are a number of control strategies that can be tried to control this pest. Since the adult beetles overwinter in the soil, rotate to a new garden location if possible and use floating row covers to keep the adults off the plants and prevent them from laying eggs. Be careful though, floating row covers can be a problem in our winds, so consider low tunnels instead. Watch for the yellow egg clusters and crush them and the adults and larvae when you can find them. You can purchase and release predatory wasps (Pediobius faveolatus) to help keep the beetle larvae under control. Remove and bag badly infested plants to get them out of the garden. After a week, remove and discard the dead plants, beetles and larvae. Plant an early trap crop of beans to attract the adults that overwintered. Once the adults attack the trap crop, pull up the plants, bag them and get rid of them. Finally, there are some insecticidal options to consider including spinosad, neem oil or pyrethryn. Be sure to follow label instructions and avoid spraying when pollinating insects are present.
Squash Vine Borer
The squash vine borer (Melitta curcurbitae) is the larvae of a clearwing moth that looks something like a wasp. The adults overwinter in the soil and emerge in late June or July. The females lay single eggs on the base of squash and pumpkins but they may also feed on other cucurbits. After about a week, the egg hatches and the larva burrows into the plant where it begins to feed, hollowing out the stem as it goes. Infested stems become swollen as the larva increases in size and does more damage. Later on, when it gets hot and dry, these vines will usually wilt because the feeding of the larva has destroyed a major portion of the vascular tissue.
Management of this pest is difficult. Since they often attack summer squash, larger fruited squash and pumpkins, avoid planting those in your garden. Instead, consider planting butternut squash, cucumbers, melons, and watermelons, which are less often attacked. Watch for and listen for the adults in June so that you know when other control strategies might be used. Floating row covers or low tunnels can be useful but must not be used when plants are flowering, or else you will limit pollination. Also do not use row covers if the current planting of squash or pumpkins is very close to last year’s planting if it was infested, since the adults overwinter in the soil and will emerge inside your row cover. Use yellow soapy water traps to monitor for the adults. Inexpensive yellow flying disks work well, just place them upside down, filled with the soapy water on the ground, near the squash and check them every day for presence of the adults. Watch for signs of infestation. If you see the swollen stems or petioles, use a sharp knife to slice open the stem and kill the larva. Covering up portions of the stem at nodes with moist soil can encourage new roots to form and possibly save a portion of the plant. Otherwise, remove infested plants and destroy them to kill the larva. Finally, several insecticide options like carbaryl, permethrin, bifenthrin, and esfenvalerate can be applied to the stem bases of the plants to protect the plants from the larvae as they try to burrow into the plants.
Septoria leaf spot on tomatoes
An airborne fungus, Septoria leaf spot (Septoria lycopersici) is most active in wet years when the temperatures are between 59°F and 80°F. However, some gardeners are experiencing this disease, particularly if they are overhead watering their plants. The spores need a film of moisture on leaves to germinate. Frequent rain, high humidity and watering late in the day all provide the needed moisture.
The infection first shows up as brown spots ringed by yellow, on the lower leaves. As it progresses the spots will join into larger spots and stem develops lesions. Control is best done by watering in the morning so plants dry out before dark and using drip hoses or watering just the base of the plant will prevent foliage from getting wet. While this is not a soil borne fungus, spores do fall to the ground and may be in debris from a previously infected plant. Mulch will prevent any spores on the ground from splashing up onto the plants.
If plants are frequently infected begin spraying a fungicide two weeks before you normally see the infection, and continue spraying weekly through the growing season. Copper soap is an organically approved fungicide providing good control. Protective fungicides containing chlorothalonil (Daconil®) can help with all of these diseases but they need to be applied at the first signs of the disease. Plants with a heavy infestation should be removed and put in the trash.
Powdery mildew on cucurbits
Higher humidity helps to foster powdery mildew on many different kinds of plants, including squash and other cucurbits. The fungus Podosphaera xanthii initially causes yellow spots on the foliage which later develop the typical whitish, powdery appearance that can spread over the surface of the leaves, petioles, stems and even the fruit. The powdery growth of the fungus restricts photosynthesis so it weakens the plant and reduces production. It is generally more severe in plantings that are densely planted, heavily infested with weeds, shaded or if the plants are overly fertilized with nitrogen fertilizer.
Best management practices include avoiding the planting practices described above that foster the disease. Look for disease tolerant or resistant varieties and monitor for the first signs of the disease so you know when you might choose to initiate a fungicide treatment regimen utilizing a spray that contains sulfur.