Breakdown of Cross-Protection of Grapefruit from Decline-Inducing Isolates of Citrus tristeza virus Following Introduction of the Brown Citrus Aphid.

A 21-year-old replicated field planting of 84 'Ruby Red' grapefruit trees cross-protected with three mild isolates of Citrus tristeza virus (CTV) was assessed for decline-inducing and non-decline-inducing isolates of the virus 5 years after the brown citrus aphid (BrCA) (Toxoptera citricida Kirkaldy) first was established in the experimental area. Prior to the introduction of the BrCA, the cross-protecting mild isolates had significantly reduced detectable infection with decline-inducing isolates of CTV for 16 years (average infection of 13% in cross-protected trees compared with 67% in unprotected trees). After the introduction of the BrCA, infections with decline-inducing CTV (measured by enzyme-linked immunosorbent assay) were 57, 81, and 71% for trees protected with three mild isolates, respectively, compared with 95% in unprotected trees. These results suggest that the introduction of BrCA accelerated the breakdown of cross-protection against decline-inducing isolates of CTV in grapefruit.

Cross-Protection of Grapefruit from Decline-Inducing Isolates of Citrus Tristeza Virus.

The ability of three mild isolates of citrus tristeza virus (CTV) to prevent natural infection of 84 Ruby Red grapefruit on sour orange rootstock by aphid-transmitted, decline-inducing isolates of CTV was assessed by symptoms and verified by enzyme-linked immunosorbent assay (ELISA) after 16 years. Of 21 trees in each of four treatments protected by the DD 102 bb, Guettler HS, and DPI 1-12-5-X-E mild CTV isolates, 14, 10, and 14% were infected by severe isolates (MCA13 monoclonal antibody reactive) compared with 67% for unprotected control trees. The health of trees protected by the DD 102 bb CTV isolate was significantly better than that of unprotected control trees as measured by decline, tree ratings, and tree height. These data suggest that infection by certain mild isolates of CTV can cross-protect grapefruit trees on sour orange rootstock from decline-inducing isolates of CTV that are prevalent in the Indian River region of Florida.

A Psorosis-Like Agent Prevalent in Florida's Grapefruit Groves and Budwood Sources.

Rio Grande Gummosis (RGG) is one of the most serious diseases of grapefruit in the Indian River region of Florida. Experiments were conducted to determine if a graft-transmissible agent was associated with this disease in commercial grapefruit, and if any such agent was present in scion sources used for grapefruit propagation. Indexing data from indicators revealed that 79% and 31% of the commercial grapefruit and scion trees, respectively, contained a psorosis-like agent (PLA). The PLA was present in both RGG-affected and symptomless trees. We conclude that Florida's grapefruit propagation sources are contaminated with a PLA that is now widespread in commercial plantings.

Natural Field Spread of Mild and Severe Isolates of Citrus Tristeza Virus in Florida.

The effectiveness of five different aphid control regimes in delaying movement of mild and severe isolates of citrus tristeza virus (CTV) into a CTV-free sweet orange scion on sour orange rootstock block was monitored annually for 5 years, using severe isolate-specific and isolate-nonspecific monoclonal antibodies. The average percentage of trees infected with a severe isolate of CTV was 32, 32, 20, 25, and 28 for trees treated with Temik, Temik + Meta-Systox R (MSR), MSR, stylet oil, or no aphid control, respectively, at the conclusion of the experiment (5 years). These percentages were not significantly different (P ≤ 0.05). The average percentage of trees infected only with mild isolates was 13, 10, 7, 7, and 17 for the above treatments, respectively. The infection with mild isolates was significantly greater (P ≤ 0.05) with no aphid control than with stylet oil or MSR treatments.