In the Garden:
Southwestern Deserts
September, 2006
Regional Report

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This grapefruit tree's oozing gummosis may have been caused by improper watering that allowed the cracked, sunburned bark to sit in water.

Citrus and Phytophthora

Healthy citrus trees are multipurpose performers in our low-desert landscapes. They provide sweet fragrance when in bloom; tasty fruit over a long harvest season; and dense, glossy foliage that creates a terrific privacy screen year round.

Unfortunately, citrus trees are often neglected or harmed by poor cultural practices just because that's the way the previous owner set it up. Although the trees keep plugging along and provide a crop, they aren't as attractive as they could be, fruit may be inferior, and eventually the trees succumb. Gone is that glorious winter box of navel oranges and pink grapefruit for shivering relatives in cold climates. Here are some tips for maintaining vigorous trees.

Watering
Effective watering is key. Water should penetrate deeply through the tree's root system. For mature trees (in the ground four or more years) that means 3 feet deep. Newly planted trees can be watered about 2 feet deep, gradually increasing to 3 feet as they mature. Deep watering leaches salts below the root zone, so trees don't suffer salt burn, which shows up as yellowing and browning on leaf margins. If you have flood irrigation, deep watering isn't a problem. If you use drip, bubblers or a hose, make sure the water runs long enough to soak deeply.

Mature citrus trees require a lot of water, and many people surround trees with a berm and fill it, allowing water to stand next to the trunk. This isn't good. It's imperative that water not contact a citrus tree's trunk because it is highly susceptible to a fungus called Phytophthora, which lives everywhere in the soil, moving with moisture. It enters citrus trees at the bud union (where the variety is grafted to the rootstock) or where bark is sunburned, cracked, frost-damaged, or nicked by weedwackers and mowers. Build a berm 1 foot away from the trunk to keep water out. If you choose, build a second berm farther out to contain the water. But remember to expand the outer berm as the tree grows. Water should be applied at the outer edges of the tree's canopy where feeder roots can absorb it.

Signs of Phytophthora
Common names of this disease include brown rot or foot rot gummosis because of a brownish, oozing gum; and roots may have brown or black lesions. Peeling bark below the bud union and obvious dieback in the branches are signs. Phytophthora is aggressive and can very quickly kill a tree, sometimes within a year. Your best bet is to prevent it with good cultural practices because fungicide treatment is not a sure thing.

Prevention
Purchase a tree with a bud union at least 6 inches above the soil line of its pot, and then plant so the bud union remains 6 inches above. If you're starting with a new tree, allow the lower branches to remain on the trunk as they do in the orchards. They shade the trunk so there's no need to apply white paint for protection. (Bonus: Fruit on low branches is easier to pick!) Don't do clean-up pruning in summer. Removing limbs and foliage opens up bark tissue to sunburn. Sunburned tissue cracks and peels.

Fertilize
Citrus trees are heavy feeders so don't let them stress for nutrients. Apply one-third of a tree's annual nitrogen requirement in Jan./Feb., Apr./May, and Aug./Sep. If you haven't fed your citrus with its third meal of the year, now's the time!


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