Controlling Sooty Mold

From the garden email bag, Gus from Lodi wants to know: “We have a great crop of oranges now despite the fact that many of the fruit and leaves are coated with a black substance that is easily washed off of the fruit. The fruit tastes great. Can you help? Thanks.”
 

 

That substance may be sooty mold, which is a fungus that results from the excrement of sucking insects such as aphids or scale.

 

According to the UC Davis Integrated Pest Management experts, sooty molds do not infect plants but grow on surfaces where those excrement deposits accumulate. This excrement, also known as honeydew, is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect cannot completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as honeydew. Wherever honeydew lands - leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary - sooty molds can become established.

Although sooty molds do not infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age and die, causing premature leaf drop.
 

And as you pleasantly discovered, fruits or vegetables covered with sooty molds are edible. Simply remove the mold with a solution of mild soap and warm water.

To thwart any new outbreaks of sooty mold, do not overprune, overfertilize or overwater your pest-prone plants, says the University of Florida Ag Service. These bad habits can create weak, succulent new growth on the plants, a delicacy for sucking insects.

Beginning of sooty mold. Note the small white mass of scale on the stem.

To control sooty mold, you have to control the pests. On citrus, soft scales, such as cottony cushion scales, are the usual culprits. This scale can be controlled with a light horticultural oil when the scale are in their crawling stage.

Cottony cushion scale on mandarin branch

Sooty mold can attack any plant that may have an infestation of other sucking insects, especially aphids, leafhoppers, psyllids, and mealybugs.

The UCD IPM pros also advise against using insecticides containing the active ingredient imidacloprid for controlling this pest. Although scale is listed on the label as a pest that it can control, it is not effective against cottony cushion scale. 

Photo courtesy UC IPM

 To complicate matters, imidacloprid will reduce populations of the beneficial insect, the vedalia beetle. Both the adults and larvae of the Vedalia beetle feed exclusively on the cottony cushion scale on a variety of plants including rose, acacia, magnolia, olive, and citrus.

One product that can help control many of these pests, while washing off the sooty mold from leaves or fruit, is insecticidal soap. Even a blast of water from the hose can help wash off the sooty mold without harming any beneficial insects that might be trying to help you out. Horticultural oils or Neem oil can also suppress scale populations.

Also, look for ants crawling up and down the tree. Control the ants, and you can control the pests...as well as the sooty mold. Ants herd and protect sucking insects from beneficial insects, in order to harvest the honeydew for the ant colony.

Baits containing boric acid placed around the base of the tree will control ant populations in a couple of weeks. Sticky substances, such as Tanglefoot, although messy, can stop ants. See this previous post on ant control.

 

For plastic or painted surfaces that might have sooty mold, the USDA recommends this cleaning solution:

1/3 cup powdered household detergent 

1 quart household liquid bleach

2/3 of a cup of Trisodium phosphate

3 quarts of water. 

Be sure to wear rubber gloves when cleaning with this solution.

Plants That Attract Beneficial Insects

Nature wants to make your job as a gardener as easy as possible; but you have to help. We've talked about putting in plants that attract insects whose primary job is to pollinate your garden, helping to insure a bountiful harvest of food and flowers. 

But what about attracting those other "good bugs", the crawling and flying creatures whose diet includes pests that are ravaging your garden plants? These beneficial predatory insects do not live on aphid steaks alone. They need other natural sources of food and shelter for their entire life cycle before they call your backyard a permanent home. 

What are these "Welcome Mat" plants and the beneficial insects they attract?

English Lavender

 Here is a list of those good bugs and the plants that they like to visit for shelter and as another source of food for their diet, the sugar from flowers. For some beneficials, especially syrphid flies, this nectar is necessary in order to mature their eggs. Intersperse these plants among the “problem pest areas” in your yard to attract the garden good guys.

LACEWINGS (Chrysopa spp.)
Beautiful, little (3/4”) green or brown insects with large lacy wings. 

Individual white eggs are found laid on the ends of inch-long stiff threads. 

It is the larvae (which look like little alligators) that destroy most of the pests. They are sometimes called aphid lions for their habit of dining on aphids. They also feed on mites, other small insects and insect eggs. On spring and summer evenings, lacewings can sometimes be seen clinging to porch lights, screens or windows.

Plants that attract lacewings:
•Achillea filipendulina    Fern-leaf yarrow
•Anethum graveolens    Dill
•Angelica gigas    Angelica
•Anthemis tinctoria    Golden marguerite
•Atriplex canescens    Four-wing saltbush
•Callirhoe involucrata    Purple poppy mallow
•Carum carvi    Caraway
•Coriandrum sativum    Coriander
•Cosmos bipinnatus    Cosmos white sensation
•Daucus carota    Queen Anne’s lace
•Foeniculum vulgare    Fennel
•Helianthus maximilianii    Prairie sunflower
•Tanacetum vulgare    Tansy
•Taraxacum officinale    Dandelion 

LADYBUGS
Easily recognized when they are adults by most gardeners. However, the young larvae, black with orange markings, eat more pests than the adults, and they can’t fly. Yellowish eggs are laid in clusters usually on the undersides of leaves.

Ladybug larva

 


Plants that attract ladybugs:
•Achillea filipendulina    Fern-leaf yarrow
•Achillea millefolium    Common yarrow
•Ajuga reptans    Carpet bugleweed
•Alyssum saxatilis    Basket of Gold
•Anethum graveolens    Dill
•Anthemis tinctoria    Golden marguerite
•Asclepias tuberosa    Butterfly weed
•Atriplex canescens    Four-wing saltbush
•Coriandrum sativum   Coriander
•Daucus carota    Queen Anne’s lace
•Eriogonum fasciculatum   CA  Buckwheat
•Foeniculum vulgare    Fennel
•Helianthus maximilianii    Prairie sunflower
•Penstemon strictus    Rocky Mt. penstemon
•Potentilla recta ‘warrenii’   Sulfur cinquefoil
•Potentilla villosa    Alpine cinquefoil
•Tagetes tenuifolia    Marigold “lemon gem”
•Tanacetum vulgare    Tansy
•Taraxacum officinale    Dandelion
•Veronica spicata    Spike speedwell
•Vicia villosa    Hairy vetch

HOVERFLIES

Also known as syrphid fly, predatory aphid fly or flower fly. Adults look like little bees that hover over and dart quickly away. They don’t sting! They lay eggs (white, oval, laid singly or in groups on leaves) which hatch into green, yellow, brown, orange, or white half-inch maggots that look like caterpillars.

They raise up on their hind legs to catch and feed on aphids, mealybugs and others.

Plants that attract hoverflies:

Achillea filipendulina

•Achillea filipendulina    Fern-leaf yarrow
•Achillea millefolium    Common yarrow
•Ajuga reptans    Carpet bugleweed
•Allium tanguticum    Lavender globe lily
•Alyssum saxatilis    Basket of Gold
•Anethum graveolens    Dill
•Anthemis tinctoria    Golden marguerite
•Aster alpinus    Dwarf alpine aster
•Astrantia major    Masterwort
•Atriplex canescens    Four-wing saltbush
•Callirhoe involucrata    Purple poppy mallow
•Carum carvi    Caraway
•Chrysanthemum parthenium    Feverfew
•Coriandrum sativum    Coriander
•Cosmos bipinnatus    Cosmos white sensation
•Daucus carota    Queen Anne’s lace
•Eriogonum fasciculatum    CA Buckwheat
•Foeniculum vulgare    Fennel
•Lavandula angustifolia    English lavender
•Limnanthes douglasii    Poached egg plant
•Limonium latifolium    Statice
•Linaria vulgaris    Butter and eggs
•Lobelia erinus    Edging lobelia
•Lobularia maritima    Sweet alyssum white
•Melissa officinalis    Lemon balm
•Mentha pulegium    Pennyroyal
•Mentha spicata    Spearmint
•Monarda fistulosa    Wild bergamot
•Penstemon strictus    Rocky Mt. penstemon
•Petroselinum crispum    Parsley
•Potentilla recta ‘warrenii’   Sulfur cinquefoil
•Potentilla villosa    Alpine cinquefoil

Rudbeckia

•Rudbeckia fulgida    Gloriosa daisy
•Sedum kamtschaticum    Orange stonecrop
•Sedum spurium    Stonecrops
 



•Solidago virgaurea    Peter Pan goldenrod
•Stachys officinalis    Wood betony
•Tagetes tenuifolia    Marigold “lemon gem”
•Thymus serpylum coccineus    Crimson thyme
•Veronica spicata    Spike speedwell
•Zinnia elegans    Zinnia "liliput"

PARASITIC MINI-WASPS
Parasites of a variety of insects. They do not sting! The stingers have been adapted to allow the females to lay their eggs in the bodies of insect pests. The eggs then hatch, and the young feed on the pests from the inside, killing them. After they have killed the pests, they leave hollow “mummies.”
 

Braconid wasps (pictured, left) feed on moth, beetle and fly larvae, moth eggs, various insect pupae and adults. If you see lots of white capsules on the backs of a caterpillar, these are the braconid cocoons. Leave the dying  caterpillar alone!

Ichneumonid wasps (pictured, left) control moth, butterfly, beetle and fly larvae and pupae. 

Trichogramma wasps (pictured, right) lay their eggs in the eggs of moths (hungry caterpillars-to-be), killing them and turning them black.

Plants that attract parasitic mini-wasps:
•Achillea filipendulina    Fern-leaf yarrow
•Achillea millefolium    Common yarrow
•Allium tanguticum    Lavender globe lily

Dill

•Anethum graveolens    Dill
•Anthemis tinctoria    Golden marguerite
•Astrantia major    Masterwort
•Callirhoe involucrata    Purple poppy mallow
•Carum carvi    Caraway
•Coriandrum sativum    Coriander
 

•Cosmos bipinnatus    Cosmos white sensation
•Daucus carota    Queen Anne’s lace
•Foeniculum vulgare   Fennel
•Limonium latifolium    Statice
•Linaria vulgaris    Butter and eggs
•Lobelia erinus    Edging lobelia
•Lobularia maritima    Sweet alyssum - white
•Melissa officinalis    Lemon balm
•Mentha pulegium    Pennyroyal
•Petroselinum crispum    Parsley
•Potentilla recta ‘warrenii’   Sulfur cinquefoil
•Potentilla villosa    Alpine cinquefoil
•Sedum kamtschaticum    Orange stonecrop

Zinnia

•Sweet alyssum - white
•Tagetes tenuifolia  Marigold - lemon gem
•Tanacetum vulgare    Tansy
•Thymus serpylum coccineus   Crimson thyme

•Zinnia elegans    Zinnia - 'liliput'

TACHINID FLIES

Parasites of caterpillars (corn earworm, imported cabbage worm, cabbage loopers, cutworms, armyworms), stink bugs, squash bug nymphs, beetle and fly larvae, some true
bugs, and beetles. Adults are 1/3 to 1/2 inch long. White eggs are deposited on foliage or on the body of the host. Larvae are internal parasites, feeding within the body of the
host, sucking its body fluids to the point that the pest dies.
 
Plants that attract tachinid flies:
•Anthemis tinctoria    Golden marguerite
•Eriogonum fasciculatum    CA Buckwheat
•Melissa officinalis    Lemon balm
•Mentha pulegium    Pennyroyal
•Petroselinum crispum   Parsley
•Phacelia tanacetifolia    Phacelia
•Tanacetum vulgare    Tansy
•Thymus serpyllum coccineus    Crimson thyme

MINUTE PIRATE BUGS (Orius spp.)
Tiny (1/20 inch long) bugs that feed on almost any small insect or mite, including thrips, aphids, mites, scales, whiteflies and soft-bodied arthropods, but are particularly attracted to thrips in spring. 

DAMSEL BUGS (Nabis spp.)
Feed on aphids, leafhoppers, plant bugs, and small caterpillars. They are usually dull brown and resemble other plant bugs that are pests. Their heads are usually longer and narrower then most plant feeding species (the better to eat with!).

 

BIG EYED BUGS (Geocoris spp.)
Small (1/4 inch long), grayish-beige, oval shaped) bugs with large eyes that feed on many small insects (e.g., leaf hoppers, spider mites), insect eggs, and mites, as both nymphs and adults. Eggs are football shaped, whitish-gray with red spots.
 
Plants that attract minute pirate bugs, damsel bugs and big eyed bugs:
•Carum carvi    Caraway
•Cosmos bipinnatus    Cosmos “white sensation”
•Foeniculum vulgare    Fennel
•Medicago sativa    Alfalfa
•Mentha spicata    Spearmint
•Solidago virgaurea    Peter Pan goldenrod
•Tagetes tenuifolia   Marigold “lemon gem”

More Tips to Keep Beneficial Insects 

Working in Your Yard:

• Use a wide variety of attractive plants. Plants that flower at different times of the year can provide beneficials with nectar and pollen when they need it.

• Plantings that are at least 4' by 4' of each variety work best at attracting beneficials. 
 

• A bird bath or backyard water feature not only attracts birds (another predator of insects), but also attracts beneficials.

• Pesticides can kill the good guys, too. Avoid using broad spectrum insecticides (especially those that contain ingredients derived from pyrethroids and organophosphates). Safer alternatives include horticultural oils, insecticidal soaps and products designed solely for specific pests, such as Bt (Bacillus thuringiensis) a bacteriacide for tomato hornworms.
 
• Tolerate minor pest infestations. The beneficial insects will get the memo before you do. This will provide another food source for the beneficials and help keep them in your yard.

• More information about  beneficial predatory insects: "The Natural Enemies Handbook", from the University of California Division of Agriculture and Natural Resources.

Let the garden good guys do the job first!

Aphids!

A morning stroll through the garden is always revealing. The low angle of the sun highlights plant features that you might miss later in the day: the intricately beautiful circular web of the orb spider; ripened fruit, ready for picking, reflected in the morning light; and, of course, that behemoth zucchini you missed last night while collecting vegetables for dinner.

California Buckeye 46 Heirloom Bush Beans

That early walk can also highlight glistening leaves on your vegetable plants. A closer look may show that it is not the morning dew. It's a "dew" of a different kind, shimmering in the sun: honeydew, the sticky secretions of that ubiquitous bad bug, the aphid. 

Turn over that leaf, and voila! There's a colony of aphids, busily sucking the life out of your plant, curling and yellowing the young foliage.

What's a gardener to do?

Predatory Aphid Fly Larva

First of all, take a closer look. There may be other critters on that leaf, the garden good guys: natural enemies such as lady beetles, lacewings, syrphid fly larvae and predatory aphid fly larvae. It's the larval stage of these beneficial insects that do the most good. 

Syrphid Fly Larva (lower left)

Syrphid fly larvae, for example, move along plant surfaces, lifting their heads to grope for prey, seizing them and sucking them dry and discarding the skins. A single syrphid larva can consume hundreds of aphids in a month.

Parasitized aphids

Look for the mummified skins of parasitized aphids, where small, harmless (to you) parasitic wasps have laid their eggs which then hatch...inside the aphid, and crawl out!  

Look for disease-killed aphids as well: they may appear off-color, bloated, or flattened. Substantial numbers of any of these natural control factors can mean that the aphid population may be reduced rapidly without the need for treatment. 

If you see these garden good guys at work on the aphids, resist the temptation to spray broad-spectrum pesticides (organophosphates, carbamates, and pyrethroids). These kill natural enemy species as well as aphids. And guess whose population builds up more rapidly after such an assault? Aphids, which are born pregnant! Natural enemy populations do not appear in significant numbers until aphids begin to be numerous. Another reason not to use broad-spectrum pesticides: aphids can develop a resistance to those products. Mother Nature bats last!

The weather is your friend, as well. Populations of many aphid species are reduced by summer heat in the Central Valley and desert areas.

Aphids on a rose bud in spring

Check the other plants, as well, especially those on the upwind side of the garden, a favorite location for aphids. One tell-tale sign of aphids: the presence of ants, crawling up and down the plants. Ants herd the aphids and defend them from natural predators, in order to harvest the honeydew secretions for the ant colony. Control the ants with ant baits, and you can control the aphid population, too.

What If You See Aphids, But None of the Garden Good Guys?

My favorite way to reduce aphid populations on plants is to knock them off with a strong spray of water from the garden hose. Most dislodged aphids will not be able to return to the plant, and their honeydew will be washed off as well. Using water sprays early in the day allows plants to dry off rapidly in the sun and be less susceptible to fungal diseases. I do this two or three times a week on plants that the aphids find attractive.

Insecticidal soaps and light, horticultural oils (such as Neem oil) will kill aphids on contact. But soaps and oils will kill any beneficial insect it contacts, as well. Still, because soaps and oils have no residual action, any migrating beneficials will not be effected. Remember, too, that spraying soaps or oils on a hot day (90 and above) may damage your plants. Test a small portion of the plant with such sprays first.

Stop Aphids Before They Start: Tips from the UC Davis Integrated Pest Management Website

Before planting vegetables, check surrounding areas for sources of aphids and remove them. Aphids often build up on weeds such as sowthistle and mustards, moving onto crop seedlings after they emerge. Check transplants for aphids and remove them before planting.

Where aphid populations are localized on a few curled leaves or new shoots, the best control may be to prune these areas out and dispose of them. In large trees, some aphids thrive in the dense inner canopy; pruning these areas out can make the habitat less suitable.

On a newly planted vegetable garden, covering the seedlings with row covers can thwart the arrival of aphids.

High levels of nitrogen fertilizer favor aphid reproduction. Never use more nitrogen than necessary.  Apply it in small portions throughout the season rather than all at once. Most organic fertilizers can be classified as low level nitrogen products as compared to synthetically manufactured fertilizers.

Another effective aphid control method: snip off heavily infected branches and leaves, and put them in the trash or compost pile (if it is a hot compost pile!).

Aphids will always be with us. But there are plenty of garden good guys out there willing to help you in the battle...if you give them a fighting chance.

Tomato Troubleshooter: 2010 Update

Tomatoes - and tomato plant problems - abound this time of year. And 2010 is turning out to be an especially vexing year for tomato growers here in California, due to our cool, wet spring. And now that the summer heat has returned, many tomato problems have returned to the forefront of gardeners' attention. 

Good sources of information to help you solve your tomato travails are the books from the University of California Division of Agriculture and Natural Resources, "Pests of the Garden and Small Farm" by Mary Louise Flint and "The California Master Gardener Handbook," edited by Dennis Pittenger.

These books take an integrated pest management approach to solving plant woes. Instead of first reaching for harsh chemical controls, the books suggest adopting cultural, physical, mechanical and less toxic methods to solve the problems. Some of these solutions border on common sense, such as not planting tomatoes in too much shade; ripping out and destroying problem plants; or, trapping snails or earwigs at night beneath boards, then destroying them the following morning. 

Here are a few of the tomato problems you've been asking about this year, along with possible solutions, that can be found in greater detail in those publications:

Sunscald

Symptoms: Tomato fruit turns light brown and leathery, on side exposed to sun. 

Possible culprit: Sunscald (sunburn). 

Solution: Don't prune leaf cover from plant. Keep plant vigorously growing to produce more leaves.

SOLAR YELLOWING. According to the  UC Davis, Vegetable Research and Information Center , the reason for the yellow or yellow-orange color, rather than the normal red, is that the red pigment (lycopene) fails to form above 86 degrees (F). This phenomenon was first described by researchers in 1952 and was later confirmed by others. When lycopene fails to form, only carotenes remain for fruit color. An orangey-red color results. In production areas where temperatures do not exceed 85 degrees (F), much higher red color develops.

TOMATO FRUIT CRACKING. Radial cracking occurs during rainy periods, when rains follow long dry periods. Fruit exposed to the sun may also develop cracks. Maintain a uniform water supply through the use of drip irrigation or mulches. A full leaf canopy will also help protect fruit from the sun and reduce cracking.
 
Tomato Hornworm or Tobacco Hornworm
Symptoms: Tomato leaves eaten, areas of fruit eaten, black, pellet-like droppings on and below plant.  

Possible culprit: Tomato hornworm. 

Solution: Hand pick in evening or morning. Use sprays that contain Bacillus thuringiensis, a less toxic microbial pesticide, to control young hornworms.

 

 Blossom End Rot

Symptoms: Tomatoes are brown or black on the bottom end. 

Possible culprit: Blossom end rot. Due to a calcium imbalance, brought on by cycles of too much or too little water; other factors that may contribute to blossom end rot include improper soil pH (tomatoes prefer soil in a range of 6.0-7.0) and too much nitrogen fertilizer.

Solution: Maintain even soil moisture. Avoid varieties most prone to it.

Root Knot Nematodes

Symptoms: Poor vigor, reduced yields, tomato leaves turn brown from the bottom up. Swellings on the roots. 

Possible culprit: Root knot nematodes. 

Solution: Plant resistant varieties, labeled "VFN". Rotate crops. Solarize soil. Add compost.

 

Root Rot

Symptoms: Tomato plants grow slowly and wilt. Roots have water-soaked areas that turn brown and dry up. 

Possible culprit: Phytophthora root rot, common in overwatered clay soils. 

Solution: Avoid heavy watering for long periods. Water more frequently for shorter periods. Pull out and discard plant if problem persists.

Fusarium Wilt

Symptoms: Tomato plants turn yellow on one side, spreads to rest of plant. Inside of main stem at base is dark red (stem on right) instead of a healthy ivory color (stem on left). 

Possible culprit: Fusarium wilt. 

Solution: Plant resistant varieties, labeled "VFN". Rotate crops. Solarize soil. Remove old plants.

 

Verticillium Wilt

Symptoms: Older tomato leaves yellow, beginning between main veins. Internal stem is slightly tan colored, in small patches. 

Possible culprit: Verticillium wilt. 

Solution: Plant resistant varieties, labeled "VFN". Rotate crops. Solarize soil. Remove old plants.
 

RAIN-RELATED TOMATO PROBLEMS:

BACTERIAL SPECK: Hits during rain or overhead irrigation early in the season, during cool weather. Retards growth, reduces yields up to 25%, fruit spots, leaf spots. Leaf spots are near the edge of the leaf, dark brown with a yellow ring. Can spread throughout the leaf margin area. Common in cool coastal areas of CA. Solution: plant later in the season; avoid overhead watering.

BLACK MOLD: More common in the late season, when rain or free water affects ripening fruit. Can be cosmetic, or cause deep lesions. Pick fruit as it ripens; keep fruit surrounded by leaf canopy.

 

LATE BLIGHT: Develops during spring rain or overhead irrigation when air temperatures are near 70. The fungus overwinters on nearby areas where potatoes or tomatoes may have been planted. Purple-brown areas on leaves, turn to brown. Fruit may brown, but stays firm. Avoid sprinkler irrigation in cooler weather, especially if potatoes are nearby. Do not use sprinklers on bush-type tomatoes that have developed a dense canopy. Clean up and discard crops after harvest.

Tomato Cracking
  Can be rain or sun related. Maintain a uniform water supply through the use of drip irrigation or mulches. A full leaf canopy will also help protect fruit from the sun and reduce cracking.

Other problems that may be attacking your tomatoes this summer include whiteflies (control with insecticidal soap), aphids (ditto), powdery mildew (give plants full sun, regular watering and avoid excess fertilizer; organic vegetable fertilizers are a better choice), tomato russet mites (don't plant tomatoes near petunias or potatoes) as well as cosmetic tomato problems such as cat facing (puckering of fruit caused by cool weather when plant bloomed) and concentric circles near the stem (due to sudden, rapid growth).