How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Stubborn disease is endemic in the warm inland growing areas, where it affects primarily sweet orange, grapefruit, and tangelo trees. The disease is more of a problem in young orchards than in mature groves. The pathogen is a phytoplasma, which is spread by leafhopper (primarily beet leafhopper) feeding, and by grafting and budding. Treatment of leafhoppers in the field does not prevent the spread of the mycoplasma.

The most obvious symptoms of stubborn-infected trees are a low yield of abnormally small fruit, the absence of fruit, and the stunted, feathery growth of the canopy. The leaves are small and grow upright close to the stems. Symptoms are typically similar to zinc deficiency. The trees usually develop unseasonal growth flushes and blossoms. The few fruit produced remain small and are lopsided. The best way to see the off-centered navel and uneven sides is to cut a fruit in half.

Certain other fruit symptoms may appear. Depending on the ripening stage of the fruit, you may see stylar end greening; the blossom end of the fruit remains green while the stem end becomes colored. Fruit of seedy cultivars have dark-colored, small seeds aborted early in their development. The fruit may have an insipid or bitter flavor; on some cultivars, they also become acorn shaped.

If young trees are infected, the entire tree may remain small and unproductive. If mature trees become infected, a single branch may show symptoms, and the disease may or may not spread slowly throughout the tree.

COMMENTS ON THE DISEASE

Stubborn disease is often difficult to diagnose, especially in the early stages of disease development when symptoms are subtle or when other disorders are present. Severity of symptoms may vary among citrus species. Trifoliate and trifoliate hybrids, as well as lemons and limes, appear more tolerant. Stubborn disease does not kill trees, but stunts growth and inhibits fruit production. Because hot, dry weather favors the development and spread of the stubborn pathogen, it has become a problem in the San Joaquin and desert valleys.

MANAGEMENT

Management of stubborn disease focuses on preventing the disease and avoiding its spread. Preventative measures mainly apply to nursery practices, such as maintaining stubborn-free mother trees for budwood. Grafting budwood onto indicator seedlings or culturing leaf and fruit samples in the lab can determine the presence of the stubborn organism. No commercial laboratories, however, are currently equipped to carry out these tests. In an established orchard, observe the trees carefully for any signs of stubborn disease in late fall or early winter. A sparse crop, a useful diagnostic symptom, becomes apparent as fruit color changes to orange. Map or flag the trees suspected of being infected and recheck the orchard several times during the year to confirm your diagnosis.

Cultural Control
When planting an orchard, obtain trees from an area that does not have a high incidence of stubborn disease. Replace diseased and unproductive trees. Topworking is not advisable because the pathogen moves freely between the scion and rootstock.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:
J. A. Menge, Plant Pathology, UC Riverside
H. D. Ohr, Plant Pathology, UC Riverside

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