How to prevent watermelon mosaic

Spread predominantly by aphids and occasionally leaf miners, watermelon mosaic virus can also be carried on garden tools and clothing, so sanitize your tools regularly. The virus is only able to survive inside aphids for a few hours, so creating physical distance between potential carriers of the virus can also reduce infection. Crop rotation and removing infected plants can break this disease triangle.


Weeds, such as lambsquarters, cheeseweed, goosefoot, and Russian thistle, can act as vectors for this disease, so keep them away from your watermelon and other susceptible plants.


Horticultural oil spray can also interrupt transmission of this virus, but may cause problems of its own.


Insecticides are not effective because the disease is transmitted before the chemicals can kill the carrier. You can use reflective mulches under susceptible plants to repel aphids. If you use reflective mulch, be sure to remove it before the summer sun uses it to cook your plants.

Few plants are as productive as summer squash. They grow quickly, provide a continuous harvest, and they shade the ground under their prickly leaves, making it an excellent foodscape plant.


Summer squash vs. winter squash
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All squash plants are cucurbits, along with gourds and cucumbers. Squashes are classified as either summer or winter varieties. The main difference between summer squash and winter squash is when it is eaten. Summer squashes, which are generally eaten immature, have thin, tender skins, while winter squashes have hard skins that allow them to be stored for longer periods of time. Common winter squashes include pumpkins, Hubbard, acorn, and butternut squash.


Types of summer squash

There are two main types of summer squash: zucchini and yellow, but these are divided up into several different varieties:

• green zucchini - the classic
• yellow (or golden) zucchini - no tapering, straight cylinder
• Romanesco zucchini - stays tender even when it reaches horse leg proportions
• straightneck yellow - tapers at the stem end
• crookneck yellow - tapers at the stem end with a crook
• pattypan - scalloped disks
• cousa - shorter and stubbier than zucchini
• zephyr - a cross between yellow crookneck and yellow acorn
• eight ball - a spherical zucchini

How to grow summer squash

Summer squash prefers a sunny, well drained area, with a pH of 5.5 to 6.5, so acidification may be needed. Squash seeds are very large and should be planted about an inch deep in May and June. [The basic rule of thumb for the planting depth for any seed is to use the longest length measurement of the seed and bury it that deep.] Keep the soil moist until germination occurs, but do not let it stay soggy or damping off disease may kill your squash seedlings. Squash plants generally do not take kindly to transplanting, so it is better to plant the seeds where you want them in the first place.


​These plants will take up some space, though they can be grown in containers, towers, and straw bales, and they love raised beds! Squash is a traditional member of the Three Sisters Method of growing beans and corn in a mutually beneficial garden design. Generally, winter squash vines can get very long, while most summer squashes have more of a mounding growth, though this isn’t always the case. Be sure to read the seed packet for variety-specific information. Squash is a relatively light feeder, so fertilizer is rarely needed. These plants generally get enough nitrogen from the soil. Too much nitrogen will encourage plenty of leaves but very little fruit. Of course, adding some aged compost around the plants as mulch certainly wouldn’t hurt!


Summer squash pests and diseases

If you look at a list of all the pests and diseases that affect summer squash, you’d wonder how these plants survive at all. But they do. In fact, they thrive! But it’s always a good idea to know what might happen, so you can nip it in the proverbial bud before things get out of hand. Common summer squash pests include aphids, armyworms, cutworms, redhumped caterpillars, leaf miners, loopers, cucumber beetles, nematodes, grasshoppers, slugs and snails, stinkbugs, wireworms, earwigs, and various beetles. Squash bugs are generally your biggest threat and they make themselves known in July in the Bay Area. As far as I know, squash vine borers have not yet made their way over the Rockies, but be forewarned! Row covers go a long way toward protecting your summer squash plants against these pests. Summer squash diseases can categorized by the pathogen:

• Fungal - verticillium wilt, powdery mildew, Fusarium crown and foot rot, and belly rot
• Bacterial - angular leafspot
• Viral - cucurbit yellow stunting disorder (spread by whiteflies), squash mosaic, zucchini yellow mosaic, and curly top

Environmental conditions, such as irregular watering, can cause blossom end rot, bitter fruit, and blossom drop. Poor pollination can also be a problem. Despite all these threats, summer squash plants nearly always produce an astounding amount of food.

Squashes are members of Cucurbita. The squash family includes edible fruits that grow on vines, and inedible gourds, that are used to make containers. Both types are grown the same way. In either case, winter varieties of these Native Americans can grow up to 50 feet long! Summer squashes tend to have bushier growth than winter varieties, but most squash plants can really spread out, so don’t plant more than you have room for in the garden.

 Caring for squash plants

Since these plants take up some room, they are often grown in ‘hills’. These hills are usually 6-8” high and 12-24” wide and can support two or three plants. Squash seeds planted in rows should be 18 to 30 inches apart, depending on the variety. Squash can also be grown in containers, raised beds, straw bales, and towers. Squash plants are rugged and they thrive in our hot, dry summers. They are heavy feeders and require frequent watering, and they prefer a soil pH of 5.5 to 7.5. Digging in some aged compost or manure when creating the hills will go a long way to ensuring a good harvest. Once these plants are established, they need nothing from you but water. During the peak of summer, afternoon wilting is normal and not cause for concern. Plants will need to be watered deeply once or twice a week, depending on the weather. Mulching around plants can reduce the need for water and it  stabilizes soil temperatures.


Types of squash

Generally, there are summer squashes and winter squashes. The seasonal reference indicates when the squash is traditionally eaten, not when it is harvested. Summer squashes have a thinner, more delicate skin. Winter squashes have a hard shell that allows for long term storage. Below are the most common varieties of each type:

• Summer squash: crook-neck, scallop, straight-neck, and zucchini
• Winter squash: acorn, butternut, Hubbard, pumpkin, spaghetti

How squash plants grow

Squash plants do not respond well to transplanting, so it is best to start them where you want them. As they grow, they create both male and female flowers. These flowers are usually yellow or orange, and the male flowers tend to emerge first. These flowers are edible, but eating the flowers means no fruit. Since there will be some female flowers emerging after all the male flowers have disappeared, you can boost your crop by cutting off and bagging the male flower anthers, which hold the pollen, and applying the pollen to later female flowers with a natural hair paint brush. In the same way, if there are not enough pollinators in your area, you can hand pollinate squash flowers. In fact, research has shown that applying pollen to female flowers manually causes larger fruit that is more likely to reach maturity. Also, the seeds within that fruit germinate faster and produce larger seedlings. This is called the xenia effect. As fruit is produced, many growers place a layer of straw underneath to reduce soil contact and the potential for fungal problems.


Squash pests and diseases

As always, aphids, cutworms, earwigs, leafhoppers, grasshoppers, leafminers, mites, whiteflies, wireworms, and thrips will lust after your squash plants. (It’s really a wonder that anything grows at all, but grow it will!) Squash bugs, squash vine borers, and cucumber beetles cause the most squash problems. Cucumber beetles can also carry wilt disease. Placing row covers over seedlings and dusting plants with diatomaceous earth can reduce pest populations organically. Powdery mildew and downy mildews, anthracnose, verticillium wilt, root rot, curly top, bacterial wilt, scab, and mosaic diseases can be reduced by planting resistant varieties, avoiding overhead watering, keeping the garden clear of any diseased leaves (and not adding them to the compost pile), and using crop rotation.


Harvesting squash

Squashes should be harvested by cutting the vine with a sharp knife about one inch above the fruit. Do not twist or yank at it, as this can damage the plant. Summer squashes are best harvested young and eaten fresh. Of course, if you just can’t take another zucchini stir-fry, or you discover a specimen the size of a horse leg, you can always treat yourself with my family’s recipe for Chocolate Zucchini Cake. Don’t bother trying to freeze cubed zucchini - it doesn’t end well. Winter squash, on the other hand, can be stored in a cool, dry, shaded area, such as a garage, for several months. I’ve had butternut squash that lasted nearly two years and tasted just as sweet and flavorful as fruit from the same vine that was eaten a year before.

Healthy squash plants can produce an astounding amount of food. This makes them an excellent Plant It Forward addition to your garden or landscape.​

The Three Sisters of Native American agriculture are corn, beans, and squash.


These ‘sustainers of life’ have a rich history of folklore, spirituality, and early agriculture. They also make sense in the garden. The Three Sisters Method of growing is an example of drought tolerant companion planting that has withstood the test of time.


The Three Sisters Method was believed to have been started by the Iroquois, or the Haudenosaunee, found in the northeast region of the Great Lakes region. This collection of five nations spoke similar languages and shared agricultural information. Native Americans relied heavily on winter squash, such as pumpkins, corn (maize), and climbing (or pole) beans for both food and trade goods for several hundred years. This successful growing method spread west and south to what would become Utah, Colorado, Arizona and Mesoamerica.


Benefits of the Three Sisters Method

Planting these three sisters together allows them to benefit each other in several ways:

• corn stalks provide a climbing structure for the beans
• young beans add nitrogen to the soil
• wide prickly leaves of the squash shade the ground, stabilizing temperature, reducing evaporation and competition from weeds, and deterring pests

In some areas, a fourth plant was added to the mix. This was usually a flowering plant used to attract pollinators, such as honey bees, to increase yield. Just as the three plants benefit each other as they grow, eating them together provides fatty acids and the eight essential amino acids needed to form complete proteins.

Planting by the Three Sisters Method

Rather than planting in rows, the Three Sisters Methods calls for flat-topped mounds, 12” high and 20” wide. Several corn seeds would be planted in each mound. In some areas, rotten fish or eels would be added at the same time, to act as fertilizer. Some areas planted all three types of seeds at the same time. Others would wait until the corn was 6” tall before adding squash and beans. Seeds would be alternately planted around the corn. Two types of beans were used: common beans (Phaseolus vulgaris) and tepary beans (Phaseolus acutifolius), which are more drought tolerant.


You can create your own Three Sisters garden design using the same companion planting concepts. Since corn has higher pollination rates when planted in blocks, rather than rows, you can set aside an area of the yard, or a raised bed, as your very own Three Sisters garden.


The corn will grow up, the beans will climb the corn and add nitrogen to the soil, while the squash protects the ground with its wide leaves. Come autumn, your family will be able to enjoy a high protein meal of beans and corn with a side order of baked or steamed squash, with very little effort on your part.


Give it a try this growing season and see how well it works for you!

We’ve all heard some seeds or plants described as heirlooms and others hybrids, but what do those terms really mean?


Both hybrids and heirlooms come about through naturally occurring cross-pollination, as opposed to genetically modified organisms (GMOs), which are created in a lab using altered DNA strands.


Pre-industrial agriculture

Before agriculture became an industry, people grow a wider variety of plants for food. That biodiversity helped offset inclement weather, diseases and pests, and other threats to a failed crop and the resulting starvation. Corporate agriculture, on the other hand, feeds countless millions by generating a smaller variety of uniform plants that consistently grow at specific rates, that can be sprayed with a variety of pesticides, herbicides, and fungicides, ship well, and store well. As many of you know, taste and texture often suffer s a result. ​

Pros & cons of heirlooms

Heirloom seeds are those that have been handed down, person to person, in a specific geographical region, for a very long time. Also, heirlooms are open-pollinated, which means pollination occurs naturally, by wind, birds, animals, and insects, and not by human efforts. Heirloom varieties are at least 50 years old (some say 100 years), and many of them have been grown consistently, in the same locale, since before WWII. These plants have evolved to take advantage of local microclimates and beneficial insects. Heirloom seeds are hand selected by gardeners from the very best plants each growing season.  Many heirloom plants do not have the uniformity or long term storage capabilities of hybrids, but growers (myself included) claim that the flavor is significantly better. Heirloom crops have more variety in size and shape than hybrids, but they always grow true to their parent plants. Heirlooms are more genetically diverse, making them more durable as a species, and less susceptible to local pests and diseases. Heirloom offspring are fertile and can reproduce.


Pros & cons of hybrids

Hybrid plants are intentionally created by cross-pollinating different varieties of a species. The intention of hybridization is to take advantage of the best characteristics of each parent plant, creating what is known as hybrid vigor (heterosis). This vigor only lasts for one generation. Hybrid seeds do not grow true to their parents and they lack vigor and genetic diversity. This lack of diversity is what caused the Irish Potato Famine of the 1840s. If all the plants are identical, they are equally susceptible to pests and diseases. A single threat can be devastating.


Creating a hybrid that grows “true” to the desired characteristics takes years of diligent effort. Plants are often pollinated by hand or grown in greenhouses or pollination bags that block contamination from outside pollen to ensure that pollination only occurs between the desired plants. The majority of the fruits and vegetables you see in grocery stores are hybrids. Harvests are very consistent in size and shape. Hybridization is done for many specific characteristics:

• improved disease resistance
• easier growing
• early maturity
• better flavor
• bigger size
• longer storage capabilities

When shopping for plants and seeds, one way to know if it is a hybrid is to look at the Latin name. If you see the letter “x” between words in the name, it is a hybrid. For example:

Raspberry (Rubus idaeus) crossed with blackberry (Rubus ursinus)
  creates Loganberry (Rubus x loganobaccus)

*Check labels for the letters V, F, N, T or A. These symbols indicate a resistance to verticillium wilt, fusarium wilt, nematodes, tobacco mosaic virus or alternaria stem canker, respectively.

Understanding the difference between heirlooms and hybrids can help you make the right choice if you want to collect viable seeds from your harvest for next year’s planting.

Spring is the time of year when it is common to see white powdery patches appear on leaves. This bane of gardeners is called powdery mildew, and it can occur in autumn, as well.

Cucumber, melon and other cucurbits are susceptible to powdery mildew. You may also see it on tomatoes, roses, snapdragons, chrysanthemums, peas, artichoke, beets, grapes and wheat. 


What starts as a small white spot, powdery mildew expands to engulf an entire leaf as the nutrient-sucking fungi bleed the life from your garden. It can be found on either side of a leaf and sometimes on stems.


Powdery mildew identification

Powdery mildew is a fungal disease. It is caused by different types of fungi (e.g., Erysiphe spp., Sphaerotheca spp.), depending on which plant is affected. Contrary to common belief, moisture and humidity are not needed for these fungal beasties to appear.


The white powder seen on leaves is actually thin layers of fungal tissue (mycelium). Other symptoms of powdery mildew include:

• yellowing leaves
• leaf die off
• distorted leaves
• yellow patches on leaves (especially on tomatoes, peppers, onions and artichoke)

How does powdery mildew grow?

Powdery mildew fungi simply need living plant tissue to survive and thrive. To make matters worse, their spores are carried on the wind, so the battle never ends. The reason powdery mildew seems to disappear in the heat of summer is that these microorganisms prefer shade and temperatures between 60° to 80°F. Our California summers are simply too hot for the spores to reproduce. Instead, they remain dormant until conditions improve.


Not only does powdery mildew cause leaf loss, it can also weaken a plant. This lowers production and increases susceptibility to other pests and diseases, such as citrus blast. Leaf drop can also lead to sunburn damage.


Powdery mildew management
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Prevention and vigilance are the best ways to counteract powdery mildew. These tips can help, but nothing will eliminate powdery mildew in the garden:

• Plant resistant varieties
• Change the species of annual plantings to break the fungal life cycle
• Monitor plants for new spots of white mycelium and remove infested leaves
• Plant in sunny areas
• Provide good air circulation (use spacing and pruning)
• Avoid applying excess fertilizer
• Apply horticultural oil (not within 4 weeks of applying sulfur, or your plants will BURN)
• Apply sulphur products before symptoms appear (only when temperatures are below 90°F)
• Apply biological fungicide (not as effective as oil or sulphur)
• Apply fungicides, as a last resort

Some people recommend spraying plants with a baking soda solution, but this can add too much salt to your soil. 


More recent research shows that spraying the soil with milk before planting and then spraying the leaves of susceptible plants when the disease is most likely to start appearing can significantly reduce the incidence of powdery mildew in your garden.

You pluck an orange from a tree and a tiny white insect flies out from under a leaf. On the underside of that leaf is a strange white spiral. You have discovered whiteflies.

Whiteflies are not related to flies at all. Instead, they are close cousins to aphids, mealybugs, soft scale and armored scale. These pests cause billions of dollars in plant damage each year, sucking the sap from 500 different plant species, including your cabbage, cucumbers, eggplant, melon, squash, and tomatoes.

Whiteflies are quick and difficult to see. It is easier to tell if a garden is infested with whiteflies if you see white spirals on the underside of leaves. These spirals are whitefly eggs.


Whitefly lifecycle

Adult females walk a spiral path, laying eggs in a waxy trail that holds the eggs to the leaf. In warmer parts of California, whiteflies can breed year-round. Whiteflies can reproduce at mind-boggling rates. They go through four developmental stages, or instars. Eggs hatch out as nymphs, called crawlers. The next three stages are nearly immobile, much like scale insects. The final instar is a pupal stage.

​Whitefly control

Heavy whitefly populations are very difficult to control. Any leaves with whitefly eggs should be removed and discarded. If the leaf looks particularly healthy, you can simply rub your thumb over the eggs to destroy them. Reflective mulches repel adult whiteflies, aphids, and leafhoppers. Also, yellow sticky traps can be used to trap and monitor whitefly populations.

Natural predators, such as minute pirate bugs, green lacewings, big-eyed bugs, and hummingbirds are the best defense against whitefly infestations. You can attract these garden helpers by planting zinnias, bee balm, hummingbird bush and pineapple sage.


Whitefly population explosions are often created when gardeners apply broad-spectrum pesticides that kill off beneficial predators, or when ants are allowed free access to host trees. Ants can be prevented from protecting whiteflies by applying a sticky barrier around the trunk. Dusty conditions can also put the odds in favor of whiteflies. Washing off dusty plants with a garden hose can help reduce whitefly populations.

According to companion planting lore, whiteflies can be discouraged from an area by planting basil, mint, thyme, and nasturtium. I don’t know if that works, but those plants are nice to have, in any case.
​

UPDATE (12/24/19):

You may be surprised to learn, as I was, that plants release chemical compounds when bitten by insects or herbivores. These chemicals communicate with nearby plants, telling them to start protecting themselves against specific threats. These chemical messages also alert parasitic wasps and other beneficial insects, calling in the troops to protect the plant. 

​Researchers have recently learned that silverleaf whiteflies have developed the ability to flip this self-defense mechanism to their advantage. It works like this. Whiteflies are known to carry viruses. When a whitefly bites a plant, the plants used to defend themselves against the whiteflies. Now, they work harder to defend themselves against potential viral infection and create a more suitable whitefly habitat. This misinformation spreads to nearby plants, creating a cascade effect that can be very difficult to manage.


It’s a war zone out there!

How to grow butternut squash

Butternut squash grows best in full sun, though I have grown abundant crops under well-pruned fruit trees. This is a deep-rooted plant, making it well suited to areas experiencing drought. Butternut squash does not need particularly fertile soil and it can go weeks without watering. If larger leaves start to wilt, water deeply. There is no need to fertilize butternut squash.


Pests & diseases of butternut squash

Butternut squash is prone to root rot, so avoid overwatering. Seedlings are susceptible to damping off disease when the soil is too cold or too wet. Slugs and snails, caterpillars, and cucumber beetles will attack young plants. Later in the season, aphids, spider mites, and whiteflies can cause problems. Powdery mildew and other fungal diseases are common, but they rarely kill the plant or harm the fruit.


Cucurbit cross-pollination

This is one of those ‘partly true, partly myth’ situations in the garden. Traditional folklore warns gardeners against growing squash, cucumbers, and melons near each other. The claim suggests that cross-pollination will occur, affecting the taste of the produce. Since pollination can only happen with pollen of the same species, this cannot really happen. What can happen is cross-pollination between varieties of the same species. Cucumbers and melons are a different species, so they will not cross-pollinate with each other or other members of the cucurbit  family.


Summer squash, pumpkins, gourds, and some winter squash, all being members of the same family, can cross-pollinate. This pollination will create viable fruit identical to the mother plant. The genetic changes can only be seen in the seeds that are produced. If you save seeds, as I do, this may result in something really unique growing next year. Sometimes it will be edible and sometimes it won’t. Butternut squash will cross-pollinate with pumpkins, so you’ll want to keep them as far apart as possible. Oddly shaped fruit and poor taste are usually the result of over-fertilized soil, too much irrigation, or damp weather, and not weird cross-pollinations.


Harvesting & storing butternut squash

The more often you harvest the fruit, the more fruit the plant will produce. In the plant world, reproduction is the name of the game. Fruits can be eaten right away or allowed to dry out, or cure, for later use. Fruits can be allowed to cure on the vine or removed and stored in a cool, dry location.

Mites are tiny spiders that suck the living juices from nearly a thousand different garden plants. Being closely related to ticks, some varieties of mites also suck blood from mammals, like us! The study of mites and ticks is called acarology. ​


Mites prefer soil that is high in organic content with plenty of moisture. Unfortunately, that describes nearly all of our container plants and many of the microclimates found in Bay Area gardens. A highly adaptable critter, there are over 42,000 species of mites worldwide. Aside from nest mites, dust mites, varroa mites, and many others that attack birds, animals, and bees, respectively, common garden mites include gall mites, spider mites, and thread-footed mites.

Spider mites

Spider mites get their name because they build protective webs around eggs and feeding areas.  The most common spider mites in the Bay Area are the two-spotted spider mite, the strawberry spider mite and the Pacific spider mite. They are often found on the underside of leaves, where they pierce plant cells to feed. These mites are very small, usually less than 1 mm (0.04”) long, so they are all too easy to overlook until the damage becomes significant. It doesn’t take long for a population to develop, either. A single female can lay 20 eggs a day and live for up to 4 weeks. Since each offspring hatches within 3 days and becomes sexually active in only 5 days, a single, fertilized female and her offspring can produce millions of spider mites in a single season!


An interesting note: female spider mites have two sets of chromosomes, like we do, but males only have one. If an egg is fertilized, it will hatch female. If it is not fertilized, it will hatch as a male. Also, female spider mites are able to “decide” whether to lay male or female eggs, depending upon environmental conditions. To control spider mites, insecticidal soap is your best bet. Neem oil can also help. If chemical pesticides are used, repeated applications will be necessary (and progressively ineffective, as mites can develop resistance).


Thread-footed mites

Most thread-footed mites, also known as white mites, feed on fungi and algae, a few varieties have evolved to attack leaves. Specifically, the cyclamen mite and the broad mite are able to inject toxins that thin the cell walls of mature leaves. Damaged leaves display puckering, twisting and stunting. 


Mite control

The only known effective chemical pesticides against mite infestations are endosulfan, dicol, and ethyl bromide fumigation. Endosulfan was globally banned due to its toxicity to humans and its ability to accumulate in an environment, Dicol is considered a “moderately hazardous” pesticide, closely related to DDT, and the state of California classified ethyl bromide as carcinogenic and a reproductive toxin - not anything you want to be spraying on food plants.


​Broad spectrum pesticides do more harm than good because they also kill beneficial insects that feed on mites. You can buy predatory mites that help control mite infestations. If an infestation is discovered, sprays of water can be used to displace mites and make life harder for them. Garlic extract and oil of clove, rosemary, cinnamon, mint and others can also be effective. These natural treatments can be dangerous to plants, however, so use them carefully. The same goes for sulfur, especially on cucurbits. Observation and prevention are far easier than eradication.


Mite prevention

One of the easiest ways to avoid mite infestations is to create a quarantine area for new plants.  This protects established plants from new infestations and gives you the time needed to see if a new plant is carrying any pests or diseases. Also, proper irrigation reduces water stress in established plants, making them better able to protect themselves. Mites prefer dusty conditions, so keeping garden paths, trees, shrubs and other areas clean can significantly discourage mites. Encouraging beneficial insects, such as lady beetles, lacewing, and pirate bugs, by providing water and habitat, can significantly reduce mite populations without the use of pesticides or sprays.

Yesterday, I curb-scored 8 very nice tomato cages. While they come in a variety of sizes and shapes, the most common type are concentric circles held at different heights, usually with three legs.