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The Delights of Dill
San Diego County 2010 Garden Tours
Troubles in the Tomato Patch

Heirloom Tomatoes: Fruit with a History of Great Taste
The 2010 Master Gardener Plant Sale is set for June 19
Educators Earn Credits at School Gardening Conference
Our Community Gardens Grow But They Need Your Help. Please Sign Our Petition and make your voice heard.

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Troubles in the Tomato Patch

By Vincent Lazaneo

When spring arrives, gardeners anticipate eating juicy ripe tomatoes picked fresh from the garden, as they set out young plants in freshly-tilled soil. In a matter of weeks, the small seedlings grow into tall vines covered with dark green leaves, clusters of yellow flowers and marble-sized green fruit. The plants may appear healthy but a bountiful harvest may not be realized if the plants are attacked by destructive pests or diseases. 

Inspect plants regularly and when you detect pests or diseases, take steps to control them before they cause serious damage. Tomato horn worms and fruit worms can safely be controlled with a product containing the bacterium, Bt (Bacillus thuringiensi). Spider mites and soft-bodied insects like whiteflies and aphids can usually be controlled with a forceful blast of water or with sprays containing insecticidal soap or pyrethrin. These products break down quickly and are less harmful to beneficial insects than insecticides that leave a persistent toxic residue on plant foliage.

horn worm
Tomato Horn worm

Gardeners who grow tomatoes in coastal Southern California often complain that mature leaves on their plants prematurely die and turn brown. This usually occurs mid-way or later in the plant's growth cycle, and may be caused by a tiny pest, the tomato russet mite, or by a fungal disease, powdery mildew. Both are common in the San Diego region and can cause severe damage to plants if not treated early.

Russet mites are too small to be seen with the naked eye and their damage is usually noticed long before the mites are detected. Under a 20-power hand lens, russet mites appear as yellowish conical or pear-shaped bodies which move slowly. The mites use their mouth parts to pierce and remove the contents of plant cells on the surface of leaves and stems. Their feeding often causes the green tissue to turn a greasy bronze or russet color.


russet mite damage
Russet mite damage

Infestations usually start near the ground and steadily progress up the plant as lower leaves dry out. All of the foliage in the interior of an infested plant may eventually dry up leaving only an outer layer of green foliage.

Symptoms are typically observed when green fruit on standard tomatoes is about 1 inch in size. Carefully inspect plants every week or two for russet mite damage. Look for bronzing on lower leaves and stems; then use a strong hand lens to check for mites on damaged leaves and on green leaves above them.

If russet mites are present, they can be controlled with sulfur (dust or wettable powder.) Several light applications may be required to control an infestation. Some gardeners periodically apply sulfur while plants are growing as a preventative treatment.

Sulfur is an organically-acceptable pest control material. A hand-crank dust applicator or similar device is required to properly apply dusting sulfur. It should be applied when the weather is dry and the wind is calm. Always wear goggles for eye protection.

Apply a very light coating of dust to all plant surfaces. If you see a heavy deposit of sulfur, you have applied too much.

Sulfur is also sold as a wettable powder which is much finer than dusting sulfur. It does not dissolve in water but stays suspended in solution with periodic agitation. To apply wettable sulfur, use a hose-end sprayer designed for wettable powders.  Sulfur can injure plants during hot weather and should not be applied when temperatures are above 90 degrees F. Squash and melons are damaged by sulfur and it should not be applied near these crops. Wash treated fruit before it is eaten or canned.

Tomato foliage can also be damaged by powdery mildew. Spores produced by the fungus on tomato plants and related weeds are carried by the wind to healthy plants. Infection and disease development are favored by mild temperatures and high relative humidity which are common in our coastal climate. Infected leaves develop irregular bright-yellow blotches; severely infected leaves soon die but seldom drop. A loss of foliage weakens plants and results in sunburned fruit.

To discourage powdery mildew, plant tomatoes in an area with full sun and good air circulation. Check plants regularly for signs of powdery mildew; if symptoms appear, sulfur sprays can be used to control the disease. Multiple early applications usually provide adequate control.

For more information on tomato growing, visit www.mastergardenerssandiego.org and click on Resources.

Vincent Lazaneo is urban horticulture advisor for UC Cooperative Extension.