Unlike people and animals, plants do not have an active immune system. Instead, injured or infected areas are walled off to prevent further injury or the spread of infection or infestation. Gumming refers to how a specialized sap, or botanical gum, oozes out of an injury site or canker to provide protection.

Gumming is particularly common among stone fruits, such as nectarine and almond. It also occurs in mango and citrus.

Causes of gumming

Environmental stress, mechanical injury, insect attacks, and disease can all trigger a tree to start gumming. Gumming creates a protective barrier and may push any invaders out. You can use specific details surrounding gummosis to identify the problem. For example, gummosis caused by insect infestation or mechanical injury often exhibits bits of bark or sawdust mixed in with the gum. Look for other damage around the gumming site: Do leaves look sick or chewed upon? Has the bark’s integrity been breached? Do you see discoloration under the bark near the gumming?

• Environmental conditions such as sunscald, frost damage, pruning, planting too deeply, excessive irrigation,, and  insufficient fruit thinning can all cause gumming.
• Mechanical injury by lawnmower collisions, rubbing branches, and outgrown tree supports can all lead to gumming.
• Insect infestation by peach tree borers, flathead borers, and other boring insects can cause gumming.
• Diseases such as Eutypa dieback, cytospora canker, Fusarium dieback, and bacterial spot may cause discoloration under the bark, leaf dieback, and stunting.

How to manage gumming

You can help your fruit and nut trees stay healthy by avoiding mechanical injuries, monitoring for pests and diseases, and regular feeding and irrigation. If you suspect disease has taken hold, scrape some of the bark from the area surrounding the gum. If you see discoloration or streaking, it is probably a disease that needs further attention. Removing affected branches can sometimes halt the progression of some diseases. Be sure to sanitize cutting tools between each cut with a household cleaner.

If the gum emerges from circular holes and contains insect larvae, the tree has the problem in hand. All you need to do is monitor the situation.

Who knew apples could get the measles?

Unlike the human variety, which is caused by a virus, apple measles is a symptom of manganese toxicity.

​Treating apple measles

Since apple measles is a symptom of too much manganese in the soil, you need to alter the soil chemistry to help your apple tree. Apple measles occurs most commonly in acidic soil, so increasing the soil pH with lime or other alkaline soil amendment will help reduce future damage.

Unfortunately, it is very difficult for trees to recover from apple measles, so get your soil tested every 3 to 5 years, so you know what they are dealing with - before the damage is done.

Yellow sticky sheets are often used in tandem with pheromone traps. The pheromones lure the insects closer and then the yellow sticky sheet makes them unable to fly away.

Whether you use a pheromone trap in conjunction with yellow sticky sheets or not, you can use the sticky sheets to monitor for pest populations in and around specific plants or beds.

Personally, I hang them in my citrus trees in early spring to significantly reduce leaf miner damage, and to help me know what else I happen to be up against at any given time.

​The best news about yellow sticky sheets? They’re cheap. And if you are having problems with indoors pests, you can always cut the sticky paper into a decorative shape and put it to work for your container plants!

Kumquats are members of the rue, or citrus family.

Native to south Asia, kumquats have been cultivated since the 12th century. Unlike oranges, or their giant cousins the grapefruit, kumquats are closer in size to olives. These somewhat tart fruits can be eaten whole, candied, chopped into a relish, fermented into a liquor, or cooked into a delicious marmalade.. These somewhat tart fruits can be eaten whole, candied, chopped into a relish, fermented into a liquor, or cooked into a delicious marmalade.

Kumquats can be grown in containers. Container grown kumquats can grow to 6 feet and produce fruit, if they are fertilized regularly.

Kumquat pests and diseases

Asian citrus psyllid is the most serious threat to citrus trees, as they can carry huanglongbing, a fatal disease. The diaprepes root weevil may also become a problem. Leaf miners, aphids, scale insects, and whiteflies are far more likely. Citrus diseases, such as armillaria root rot, alternaria rot, anthracnose, brown rot, citrus blast, exocortis, and phytophthora root rot may occur. Kumquats tend to be resistant to citrus canker and most citrus trees thrive in the Bay Area.

Consider adding kumquats to your foodscape this year for a lifetime of fragrance, color, and Vitamin C!

Plant a seed at the proper depth and it thrives. Plant it at the wrong depth and it dies. So, how is a gardener to know how deep to plant each seed?

The general rule of thumb for seed planting depth is to use twice the seed width or diameter. This means a seed that is 1/8” wide should be planted 1/4” deep. Too often, seed packets rely on a standard 1/4”  or 1/2” planting depth, which is not always in a seed’s best interest. ​​

A seed’s life

Once a seed reaches maturity, it is protected by a hard outer shell called its seed coat. This layer allows the seed to roll, float, fly, or pass through a digestive system unscathed and ready to germinate. Only when heat, light, and moisture levels reach the right combination can the seed coat allow moisture in, which triggers the growth of the embryo within. Hopefully, the seed is at the proper depth to grow into a healthy plant.

Planting seeds too deeply

Seeds planted too deeply are less likely to germinate or sprout. Even if they do get that far, those seedlings have to work harder to reach the soil surface. This uses up valuable energy resources that should have been available for producing leaves and stems. It also reduces their growing season.

The very tiniest seeds should simply be sprinkled on the soil surface and only dusted with a covering of soil or vermiculite. And a few seeds, such as lettuce and dill, actually need light to germinate, so they should not be covered at all.

Planting seeds too shallowly

At the other end of the planting depth spectrum, seeds planted too shallowly end up with roots too close to the surface, causing them to wither and die. Even if they don’t die, shallow roots are less able to take up water and nutrients or to anchor plants in the soil. Seeds planted too shallowly are more susceptible to temperature and moisture level changes. These plants are also more likely to bolt, rather than producing a crop.

Seed planting depth also depends on soil structure. Seeds planted in rich, loose loam or sandy soil can tolerate deeper planting than seeds planted in heavy, compaction-prone clay. 

Crops by seed depth

Here is a list of some of the more popular garden plants and their proper planting depths:

•     1/8 to 1/4 inch - celery
•     1/4 inch - arugula, broccoli, cauliflower, peppers, radish, tomatoes
•     1/4 to 1/2 inch - cabbage, carrots, eggplant, onion
•     1/2 inch - beets, cucumbers, melon, spinach, Swiss chard, watermelon
•     1 inch - pumpkin, squash
•     1 to 1-1/2 inch - soybeans
•     1 to 2 inches - beans, peas
•     2 inches - corn
•     3 to 4 inches - potatoes

Tools, such as a dibble, can help ensure that seeds are planted at the proper depth.

Finally, do not press down on the soil once a seed is in place. This compacts the soil. Instead, the act of watering will tuck your seeds in nicely while still giving them a chance to grow and thrive.

Glomalin [GLO-ma-lin]. 

You cannot see it, smell it, or taste it, but glomalin is the glue that holds soil together and the path by which nutrients move from beneficial fungi to plants.

Mycorrhizal fungi

When most of us think of fungi, we usually think of mushrooms or disease. Mycorrhizal fungi are an entirely different critter. These beneficial fungi live in soil, and in and around plant roots. They are responsible for helping over 70% of the Earth’s plants get the nutrients they need  from the soil. They do this with glomalin. Glomalin is found on the tiny hairs, or hyphae, of mycorrhizal fungi. This coating helps the fungi to retain water and nutrients as they interact with local plant root systems.

Soil

Soil is made up of 45% minerals, 1-5% organic matter, 25% water, and 25% air, on average. Soil structure tells us the size of the mineral particles, which can be sand, loam, or clay. Air and water are found in tiny spaces, called macropores and micropores. The chunks of organic matter and minerals are called soil aggregates. Those aggregates are held together with glomalin. Soil aggregates improve water infiltration, drainage, nutrient cycling, root penetration, and water retention near roots. They also help counteract soil compaction.

What is glomalin?

Glomalin is a tough, resilient glycoprotein that contains significant levels of iron. It does not dissolve in water and it is resistant to decay. Also known as glomalin-related soil proteins (GRSP), glomalin stores carbon and nitrogen, and binds mineral particles together, coating them with the same protective barrier used to protect the fungi. It is now believed that up to 30% of the carbon sequestered in undisturbed soil is held specifically by the glomalin. This is one of the reasons behind no-dig gardening, in that it reduces the negative impact on the fungi responsible for creating glomalin and helping soil hold onto that carbon.

​How does glomalin improve soil quality?

Glomalin was discovered in 1996 by Sara F. Wright, a USDA Agricultural Research Service scientist. She discovered that soils depleted of mycorrhizae and their glomalin, whether through exploitation, solarization, or fungicide use, had significantly reduced crop sizes.

Glomalin helps hold organic matter in place, improving soil aggregate stability. Soil aggregate stability is a measure of the combined physical, chemical, and biological properties of a soil sample, as well as its ability to resist degradation and erosion. Without glomalin, every drop of rain and every gust of wind would grind and disperse soil in a global Dust Bowl.

Glomalin is also what gives soil its brown color. Removing glomalin from soil leaves it a gray, rocky color.

So, what’s glomalin to you?

Recognizing the importance of mycorrhizal fungi and glomalin to soil health, you can improve plant and soil health with these tips:

• minimize the disruption of the fungal network (dig less)
• avoid pulling plants out by their roots, cut them off at ground level instead
• plant cover crops to maintain living root systems underground
• rotate nonmycorrhizal crops, such as cabbage, broccoli, and cauliflower, to give fungal networks time to repopulate an area

By keeping your soil healthy, the natural processes needed by your plants to acquire nutrients and fight disease can continue.

Zebra chip may sound like a fun new black-and-white striped snack, but it’s not. It is a bacterial disease that attacks potatoes.

Like most bacteria, Candidatus Liberibacter solanacearum doesn’t move around very well alone. Instead, it lives in the gut of potato psyllids. Potato psyllids are tiny, sap-sucking pests. As they feed, the bacteria move from the insect to the plant, infecting the vascular tissue and tubers.

Symptoms of zebra chip

There are no aboveground symptoms of zebra chip, but potato psyllid feeding causes foliage to turn yellow or purple. It can also cause pink or red discoloration of leaves. Zebra chip symptoms are only visible after you cut into a potato.

The zebra chip bacteria cause potatoes to store sugar instead of starch. That might sound like an idea for a new dessert food, but the presence of sugars causes vascular tissue to turn into ugly brown lines. When cooked, these brown lines turn black, hence the name. This condition also reduces crop size by 20 to 50%. Healthy-appearing potatoes from plants affected by zebra chip are more likely to sprout while in storage. Seed pieces taken from infected plants may not sprout at all, or they produce weak, infected plants.

Controlling zebra chip

The only way to control zebra chip is by managing potato psyllids. Yellow sticky sheets trap potato psyllids with little effort on your part. Spinosad can also be used to reduce potato psyllid populations. These treatments won’t eliminate the psyllids, but they will help. Inspect potato, bean, and pepper plants regularly for signs of psyllids.

Plum bud gall mite identification

In late winter, galls begin to form around these tiny invaders. By spring, adults emerge from their protective galls. At 1/100th of an inch in length, these mites are too tiny to see with the naked eye. If you have a 20x hand lens, you may be able to see them, if you look very closely. They can be a translucent yellow, pink, white, or purple, with two pairs of legs up near the head. You are more likely to see galls on new shoots and fruit spurs that plants produce in response to these invaders. Galls are warty, bumpy growths that don’t look like normal tissue. ​

Controlling plum bud gall mites

Treating your trees with wettable sulfur in March or April, when plum bud gall mites first start to emerge from their protective galls, has been effective in controlling these pests in other regions. Treatments may need to be repeated, depending on the level of infestation. Note that apricot leaves are very sensitive to sulfur, so you can only treat apricot trees with sulfur before leaves emerge. Because these particular eriophyid mites are new to the region, we do not yet know what sort of an impact native predatory insects will have on controlling plum bud gall mite populations.

If you happen to see this new pest on your trees, please contact your County Extension Office right away.

Plants being grown indoors, or in areas without enough bees and other pollinators, cannot set fruit without mechanical pollination. Some crops, such as cucumber, melon, pumpkin and other squash, can be coaxed into producing far more fruit if hand pollination its used, due to the timing issues related to male and female flowers occurring at different times.

If you grow plants indoors, you will need to pollinate the flowers by hand to get fruit. Container plants that are at a distance from their fellows will also benefit from hand pollination. Loquats, kiwifruit, and mangos, in particular, often require hand pollination. [Due to heavy pesticide use in China, the lion's share of all their fruit crops are now pollinated by hand.]

Hand-pollination is not difficult, but it is tedious. To better understand how hand pollination works, let’s have a quick review of flower anatomy and the pollination process.

Flower anatomy

For a more detailed description, I urge you to read my posts on flowers and pollination. In the most basic terms, flowers can be male, female, or both, but not necessarily at the same time. Male flowers have a stamen and female flowers have a pistil. The stamen consists of a pollen-producing anther at the end of a filament. Pollen tends to be yellow and sticky. The female pistil, also known as a carpel, is usually found in the center of a flower and it consists of the sticky stigma, which captures pollen, the style, a tube that leads to the ovary, and the ovary itself. ​

Pollination

As insects move around, collecting nectar and pollen for themselves, sticky pollen becomes attached to their legs and is carried from flower to flower. The pollen is captured by the stigma, enters the style, and moves toward the ovary, where fertilization occurs. If there are not enough pollinators, the pollen doesn’t get moved and we have no fruit. Unless you hand pollinate. By hand pollinating, you become the mechanism by which pollen is moved from the stamen of the male flower to the pistil of the female flower.


How to hand pollinate

There are two basic methods of hand pollination: removal of the anther, or transferring just the pollen. In most plants from the cucurbit family, the male anther is large and obvious. Without handling the part covered with pollen, simply snip off the anther, cut off or roll back the flower petals, and gently roll it around on the female pistil.

On plants with smaller flowers, such as cucumber, tomatoes, and melons, you can use a small, natural bristle painter’s brush or a cotton swab to transfer pollen from one plant to the other.

Timing is important

Male flowers tend to emerge before female flowers. Also, most flowers are only receptive to pollen for one day. Transfer pollen to freshly opened flowers, preferably in the morning. Do this every day until fruit starts to form.

Concerns about cross-pollination

This comes up every year. People worry that all members of a group, such as the cucurbits, can cross-pollinate. They can’t. Melons, squashes, and cucumbers are too different from one another to pollinate each other. That being said, varieties within a species, such as white pumpkins, Jack O’ Lantern pumpkins, and Atlantic Giant pumpkins can cross-pollinate.

Even if you have bees in your garden, you may want to try hand pollinating. Research has shown that manually applying pollen to female flowers results in 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.

Did you know that researchers at Harvard are creating miniature flying robots, called RoboBees,  to be used as pollinators?

Now you know.

What do vesicles do?

Plant cells use vesicles to move materials around, process proteins, maintain buoyancy, and all sorts of other things that we are only now learning about, though scientists have known about the existence of vesicles for a while now. What we didn’t know, until very recently, is that plant vesicles perform the same function as a type of animal cell vesicle, called an exosome, does. Their job is to take material from the interior of the cell, attach itself to the inner plasma membrane, create an opening, and then release the material into the apoplast, which includes the cell wall and the space between cells. Fungal cells do the same thing, but we didn’t know plants did until very recently. 

In animal cells, there are specialized vesicles that check the load being carried by other vesicles, to see if the contents should be destroyed or moved to the apoplast. Plant cells do not have those specialized gatekeepers, so there is still plenty to learn.

Now, this may not sound like a Big Deal, but this is how cells communicate with each other, triggering plant growth and defensive measures. In fact, exosomes are directly related to the production of defensive proteins and RNAs used to fight disease. Exosomes are also used to move those defensive proteins from nearby healthy cells to a cell under attack by a pathogen, to create protective barriers against disease, and they can even enter invading cells to inhibit their growth. [If you are interested in this sort of thing, it is called host-induced gene silencing.] On the down side, exosomes also play important roles in malignancy.

In the not-too-distant future, we may be seeing artificially generated plant exosomes crafted to boost our plants’ ability to fight disease. Similar studies are being conducted to see if plant exosomes can be used in human medicine, such as exosomes found to reduce alcohol-induce liver damage in mice, or how vesicles of the ginger plant may be able to reduce inflammation in the human digestive system. For now, I will stick with ginger tea, but maybe exosomes were the reason it has been helping all along…

Harvesting yardlong beans

Yardlong beans are generally eaten before they reach full size. This means checking every day during peak production. They taste their best when about the diameter of a pencil. If you leave them on the vine longer than that, production will slow and the beans will become tough. You can, however, allow them to get close to full maturity and then harvest the beans for food and next year’s crop. Just be careful when harvesting. Do not damage the buds from which the beans grow. These buds can produce multiple beans throughout the growing season.

You can store yardlong beans in the refrigerator for several days before adding them to stir fry, casseroles and soups. These beans do not steam well.

Yardlong bean pests and diseases

Yardlong beans are not as susceptible to bean weevils as other bean species. Ants, aphids, and thrips, however, can cause problems. The most common pests are herbivores, such as rabbits, voles, and deer.

Delicious and attractive, yardlong beans also attract pollinators. Give them a try!