In 1998, PFD caused significant damage in many areas throughout the state. Losses were severe on Navels and Valencias in some groves and, for the first time, economic damage occurred on early oranges. Inoculum increased on early season flowers in late February and early March. A general rain from March 18-20 was responsible for most of the severe damage. In 1999, there was very little rainfall during the bloom period. In a few blocks of Navel oranges, inoculum built up on some scattered winter bloom in late January and early February.
In those cases, the few rains which occurred during the main bloom caused enough damage to require fungicide applications. Very little disease was observed on other varieties. From 2000 to 2005, there again very little disease pressure. Some PFD has occurred in a few Navel orange groves in southwest and east coast groves but only a very few required applications.
We have investigated the potential for development of resistance to benomyl in the causal agent, Colletotrichum acutatum. This fungus naturally tolerates moderate levels of this fungicide, but benomyl is very effective in the field. We collected 20 isolates from each grove and determined the relative growth of each isolate on benomyl at 0.1µg/ml and at 1.0 µg/ml. We collected isolates from several groves which have been sprayed 2-3 times per year over the last 10 years, several groves which have received sporadic applications and two which have never been sprayed. In all groves, growth on media with 0.1µg/ml was about 45% and growth at 1.0 µg/ml was about 20% of fungal growth on media with no benomyl. No obvious resistant isolates have been found to date (Plant Dis. 88:125-130).
Benomyl is effective as a preventive application on flower petals before inoculation and prevents symptom development up to 48 h after infection. Applications of this fungicide after symptom development does not prevent sporulation nor does it kill spores already formed. Benomyl affects the B-tubulin protein in the spindle fibers and thus prevents mitosis and cell division. To determine the basis for the insensitivity of Colletotrichum acutatum to benomyl, we cloned and sequenced a 400-bp portion of the b-tubulin. Sequences of sensitive and resistant C. gloesporioides were compared to that of isolates of C. acutatum which are naturally insensitive (Plant Dis. 86:620-624).
A computer-assisted decision system called PFD-FAD has been developed. This system along with the PFD model and other systems for timing sprays were evaluated for several years in Brazil. PFD-FAD reduced the number of fungicide applications needed without affecting the yield.
We are attempting to determine the origin of PFD and the relationship of populations from different countries. Isolates of C. acutatum have been collected from PFD-affected flowers and from Mexican limes affected by anthracnose, from Brazil, Florida, Mexico, Belize, Costa Rica and the Dominican Republic. We are comparing these isolates to ascertain whether: (i) PFD isolates arose from lime anthracnose isolates, and if so, (ii) whether that occurred in each location or whether it occurred once and then PFD spread from a single location. Comparisons of isolates are being made using RAPDs, sequencing of the ITS region and the glyceraldehyde phosphate dehydrogenase genes.
In cooperation with the lab of K.R. Chung (CREC), we are attempting to determine the role of hormones in the development of PFD. The effect of hormone inhibitors is being determined to ascertain whether fruit loss can be reduced. A virulent mutants have been recovered to help elucidate the genes responsible for virulence and non-pathogenic isolates had a mutation in a gene for transcription activator. Pathogenicity to flower petals was retained in the mutant (Molec. Plant Pathol. 6:513-525).
