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Ft. Pierce ARC.Reserach Report FTP-1983-5 November 1983
Surveying north Florida peach and nectarine orchards for benomyl-
resistant Monilinia fructicola.
R. M. Sonoda and W. J. French
Benomyl-resistant isolates of the brown rot-inciting pathogen,
Monilinia fructicola (Wint.) Honey, were not detected in surveys of
north Florida peach and nectarine orchards in the spring of 1982 and
1983. The value of monitoring for resistance is discussed.
Monilinia fructicola (Wint.) Honey, incitant of brown.rot blossom
blight, twig blight and canker, and fruit rot is an important pathogen
of stone fruits indigenous to the western hemisphere and is also found
in Africa, Australia, New Zealand and Japan (6). Fungicides continue
to be the most important component of systems used to manage diseases
incited by this pathogen. Benomyl, introduced for commercial use in
the early 1970's was an effective fungicide against this pathogen. In
recent years, however, M. fructicola resistant to benomyl have been re-
ported from Australia (5), Michigan (1), New York (4), California (3),
South Carolina (7) and Brazil (J. M. Ogawa, unpublished). No benomyl-
I resistant M. fructicola was detected in a survey of peach and nectarine
orchards in north and central Florida in 1982 (2). About 95% of the
estimated 6500 acres of peaches and nectarines grown in Florida are lo-
cated in Madison county. The following is a report of a second survey
of peach and nectarine orchards in Florida for the presence of benomyl-
resistant M. fructicola, with a discussion of the-value of the survey.
The 1983 peach and nectarine crop in north Florida was about two
weeks later than normal. Mature to near-mature fruit with sporulating
lesions were sampled on-May 26 and May 27. The lesions were sampled by
touching their surface with a cotton swab on plastic applicator sticks.
The cotton swabs were then touched to the surface of lactic acid acidified
potato dextrose agar amended with 1 pg active ingredient (a.i.) benomyl
per ml. The plates were divided into four sectors and conidia from
four separate fruit placed on four separate locations of one petri dish.
Two hundred lesions on fruits of varieties ripe at the time of the survey,
in five orchards in Madison county, the orchard at the UF Agricultural
Research Center, Monticello in Jefferson county and the UF Horticultural
Unit in Alachua county, were sampled. Table 1 indicates location of
orchard, total area surveyed in orchard, and estimated M. fructicola
fruit rot in the orchard surveyed.
1Professors, University of Florida, Institute of Food and Agricultural
Sciences, Agricultural Research Centers, Ft. Pierce, 33454 and Monticello,
Samples were usually obtained from every fourth to seventh row
depending on the size of the orchard. Five lesions were sampled per
row regardless of row length. In some cases, five lesions were not
detected in a row sampled. The lesions sampled were usually on the
side of the tree facing the sampler as he or she walked down the row.
Every tree at the UF Horticultural unit in Alachua county was searched
for diseased fruit.
Some of the M. fructicola conidia germinated and grew on the
benomyl-amended media. When transferred to 1 pg a.i. benomyl per ml
without lactic acid, however, these isolates did not grow. When
transferred to benomyl concentrations, 0.0125, 0.025, 0.05, 0.1, and
0.2 ug per ml, the concentration reducing growth rate to 50% of that
on benomyl-free media was about 0.05 -pg per ml. This is almost iden-
tical to the base-line sensitivity of benomyl-sensitive M. fructicola
isolates collected before the use of benomyl in California (J. M.
Ogawa, personal communication). The results indicate that none of
the M. fructicola isolates collected :from Florida orchards were re-
sistant to benomyl.
Value of Monitoring for Benomyl Resistance
Early detection of benomyl-resistant M. fructicola and consequent
changeover to other fungicides will minimize losses that could occur
if benomyl was applied in an orchard with significant proportions of
benomyl-resistant isolates. Monitoring for resistance in Florida is
appropriate since benomyl-resistant M. fructicola occurs in other
areas of the U.S. including areas that ship peach and nectarines to
Florida during Florida's off-season for these commodities.
Some growers have cried "resistance" when fungicide failures due
to other causes have occurred. Three of the orchards in Madison county
(Table 1) had 'visible' amounts of brown rot. The grower of the orchard
in southwest Madison county with 2.5 M. fructicola rotted fruit per
tree (Table 1) was concerned that he had a 'resistance' problem. The
results of the survey, however, indicated that the M. fructicola iso-
lates in his orchard were still sensitive to benomyl at what is ap-
parently the base-line level. Whether early detection of resistance
will be of much more benefit than as a warning to change fungicides is
not known.at this time. The benomyl resistant M. fructicola in California
(3) appears to be as well fit for survival in the absence of benomyl as
benomyl-sensitive isolates. Once a high proportion of benomyl-resistant
M. fructicola occurs in an orchard, it may be a long time before benomyl
can be used effectively in that orchard.
When and if resistance to benomyl develops among the M. fructicola
in Florida, the initial distribution of resistant isolates will probably
be in a small area that may be missed by a survey such as the present
one. It is possible that orchards not yet surveyed may harbor benomyl-
resistant M. fructicola. Once resistant M. fructicola is detected,
surveys such as the present one will become more important as a means
of determining the continued effectiveness of benomyl for individual
Table 1. Location of peach and nectarine orchards surveyed for Monilinia
fructicola (Wint.) Honey, number of trees within area sampled and
estimated incidence of fruit rot.
Location Sample area (trees) Estimated fruit2
rot per tree
Monticello 500 0.6
North Central Madison-I 1200, 0.02
North Central Madison-II 50 2.0
North Central Madison-III 500 0.07
South West Madison-I 1000 0.8
South West Madison-II 1000 2.5
1Number of trees in.area sampled, not all trees sampled except for orchard
in Gainesville where all trees were searched for fruit rot.
2Orchards sampled were those with ripe fruit.
Sampling terminated by grower's insecticide spraying.
1. Jones, A. L. and Ehret, G. R. 1976. Isolation and characterization
of benomyl tolerant strains of Monilinia fructicola. Plant Dis.
2. Sonoda, R. M. and W. J. French. 1982. Benomyl-resistant Monilinia
fructicola not detected in peach and nectarine orchards in
Florida. Proc. Fla. State Hort. Soc. 95:130-132.
3. Sonoda, R. M., J. M. Ogawa, B. T. Manji, E. Shabi, and D. Rough.
1983. Factors affecting control of blossom blight in a peach
orchard with low level benomyl-resistant Monilinia fructicola.
4. Szkolnik, M. and J. D. Gilpatrick. 1977. Tolerance of Monilinia
fructicola to benomyl in western New York State orchards.
5. Whan, J. H. 1976. Tolerance of Sclerotinia fructicola to benomyl.
Plant Dis. Rep. 60:200-201.
6. Wilson, E. E. ahd J. M. Ogawa. 1979. Fungal, bacterial, and
certain nonparasitic diseases of fruit and nut crops in
California. Univ. Calif. Div. Ag. Sciences, Ag. Sci. Pubs.
7. Zehr, E. I. 1982. Control of brown rot in peach orchards. Plant