Stubborn Disease

Stubborn disease stunts growth of infested trees and inhibits fruit production. The most obvious symptoms of stubborn-infected trees are a low yield of abnormally small, often lopsided fruit or the absence of fruit. 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.
• The tree blossoms irregularly, resulting in fruits of varying maturity and size present on the tree.

Affected trees have branches with shortned internodes resulting in stunted, feathery appearance of the canopy. The leaves are small, sharply pointed, and grow upright close to the stems. Mottle resembling the symptoms of zinc deficiency may be present. Often there is also leaf drop and dieback. Abnormal growth pattern can lead to the characteristic flat-topped trees.

Both vegetative and fruit symptoms are often variable and irregularly distributed in the tree. 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.

The pathogen, Spiroplasma citri, is a bacterial disease, which is spread by leafhopper (primarily beet leafhopper) feeding. Control of leafhoppers in the grove does not prevent the spread of the mycoplasma, possibly because, except in young plantings, there seems to be little citrus-to-citrus transmission of the disease. Instead, it appears to come into citrus groves from alternate hosts in surrounding fields. The pathogen can also be spread by using infected propagative material when grafting and budding. It has not been shown to be either mechanically transmissible or transmitted by seed.

Because hot, dry weather favors the development and spread of Spiroplasma citri, it is endemic in the warm inland growing areas and has become a problem in the San Joaquin and desert valleys. The disease is more of a problem in young orchards than in mature groves. Severity of symptoms may vary among citrus species. It primarily affects sweet orange, grapefruit, and tangelo trees. Trifoliate and trifoliate hybrids, as well as lemons and limes, appear more tolerant.

Stubborn disease can be difficult to diagnose, especially in the early stages of disease development when symptoms are subtle or if other disorders are present. In severely affected trees, diagnosis based on visual evaluation can be quite reliable; in less severe cases of stubborn disease, positive identification often requires testing. Testing based on culturing of the bacterium from vegetative or reproductive tissue can take 2 to 3 weeks and contamination may result in false positives. Currently, a PCR test is more sensitive than the traditional culturing method because it can detect the pathogen in a sample at lower concentrations. When collecting samples for testing, take fairly large samples (about 15 budsticks) in late summer through early fall (July–October) from symptomatic areas of the tree.

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 are currently equipped to carry out these tests; however the Citrus Clonal Protection Program (CCPP) at UC Riverside can run 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.

For orchards of any age

• Obtain trees from an area that does not have a high incidence of stubborn disease.
• Use Spiroplasma citri-free budwood for all propagations including topworking, though topworking is not advisable because the pathogen moves freely between the scion and rootstock.
• Remove replants infected with stubborn disease and replant with disease-free trees.

For orchards less than 6 years old

• Replace diseased and unproductive trees yearly.
• Maintain a strict weed control program in and around orchards.
• Avoid using cover crops susceptible to Spiroplasma citri such as periwinkle, many Brassica spp. wild radish, and London rocket (Sisymbrium irio).