ASSESSING POTENTIAL OF COLLETOTRICHUM ACUTATUM WILD-TYPE AND AUXOTROPHIC MUTANTS AS BIOLOGICAL FRUIT THINNING AGENTS IN CITRUS

Authors: STOVER, E. W. - UNIV. OF FLORIDA; LIN, Y. J. - UNIV. OF FLORIDA; Rosskopf, Erin; SONODA, R. - UNIV. OF FLORIDA

Submitted to: Journal of the American Pomological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2002
Publication Date: 6/1/2002
Citation: Stover, E., Lin, Y., Rosskopf, E.N., Sonoda, R. 2002. Assessing potential of colletotrichum acutatum wild-type and auxotrophic mutants as biological fruit thinning agents in citrus. Journal of American Pomological Society. 56: 230-234.

Interpretive Summary: Reduction in cropload, known as thinning, is used to enhance profitability of several commercial tree fruits. Thinning has considerable potential value in citrus production since there is often an economic premium for larger fruit associated with moderate croploads, and some citrus cultivars become alternate bearing when they are permitted to set large crops. Some materials are registered for thinning of citrus, but may be quite expensive. Postbloom fruit drop (PFD) of citrus caused by the fungus Colletotrichum acutatum is a widely-distributed disease infecting flowers and inducing abscission of citrus fruitlets in Florida and the Caribbean, and many Central and South American countries. A typical symptom of PFD is persistent blossom basal disks and calyxes, which resemble buttons and are commonly called PFD buttons. There is a significant negative relationship between the number of PFD buttons formed and the number of fruit produced in the same bloom period, and substantial crop losses sometimes result. This suggested that the PFD pathogen C. acutatum could have potential as an economical and practical biological agent for controlling citrus cropload. To be commercially acceptable, there must be minimal risk of excessive crop loss from severe PFD in subsequent seasons, and the number of buttons from the previous season is highly correlated with PFD severity the following year. In this study we assess the potential for use of C. acutatum as a biological thinning agent by comparing a wild-type strain alongside induced auxotrophic mutants, which are likely to largely disappear from the grove prior to the next season's bloom. At harvest, there were no differences in fruit load or fruit size among trees inoculated with any of the fungi, therefore the environmental conditions needed to have the desired effect are too precise to use for this application.

Technical Abstract: Colletotrichum acutatum, causal agent of postbloom fruit drop of citrus, and two induced C. acutatum mutants (3-3 and 3-2) were tested as potential agents for reducing fruit load on Valencia (Citrus sinensis ) and 'Temple' orange (C.reticulata x C. sinensis). Wild-type C. acutatum (RST) and a C. gloeosporioides isolate were applied as conidia-suspensions while induced mutants of C. acutatum (3-3 and 3-2), producing few conidia in culture, and the wild-type RST were applied as mycelial suspensions to Valencia (1999) and Temple (2000) orange trees in bloom. Application of the wild-type isolate RST resulted in the characteristic blossom infection and persistent enlarged calyxes ('buttons') associated with postbloom fruit drop. The bloom period in 1999 was extremely dry and only the mycelial suspensions of RST resulted in significant formation of PFD buttons. In 2000, some rain occurred during bloom, and conidial suspensions of RST resulted in greater button formation than did the mycelial suspensions. No other treatments resulted in greater button formation than was observed in non-inoculated controls, and little natural PFD was observed. At harvest, there were no differences in fruit load or fruit size among trees inoculated with wild-type C. acutatum, mutant C. acutatum isolates, the C. gloeosporioides isolate and trees not receiving fungus inoculum.