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Development of Control Measures for Alternaria Fruit Rot on
Oranges in Arizona
Michael Matheron, Michael Maurer, Dean Bacon, James Truman, Al Lopez
Abstract
The incidence and severity of Alternaria fruit rot on navel oranges has
increased in Maricopa County. The objectives of this study were to test the
efficacy of two fungicides, Kocide 101 and Rovral, for disease control and to
determine an application schedule that will result in optimum control of
disease. In 1994, compared to nontreated trees, a significant reduction in the
number of dropped oranges occurred on trees sprayed one to five times with
Rovral at monthly intervals from April through August. In the same year,
monthly applications of Kocide 101 from April through August were no better
than leaving trees untreated. In 1995, Rovral treatments from March through
August provided no apparent beneficial effects on control of Alternaria fruit
rot, while a single application of Kocide 101 in December, January, or
February resulted in significantly less dropped oranges compared to
nontreated frees. Because of the inconsistent activity of Kocide 101 and
Rovral in these two studies, an additional trial is in progress during 1996. The
additional data from 1996 may help identify when and what fungicide(s) could
provide significant control ofAlternaria fruit rot of navel oranges in Arizona.
Introduction
Alternaria fruit rot is a fungal disease caused by Alternaria citri. The fungus attacks only citrus fruit and occurs
throughout the citrus producing areas of the world. Alternaria rot is mostly a problem on fruit in storage; however,
the disease can develop in the orchard, where premature fruit drop and rot of ripening fruit can occur. Diseased
fruit in the orchard color prematurely and may develop a light brown to blackish discoloration of the rind at or near
the styler end. On the other hand, some fruit show no external evidence of infection and must be cut open to reveal
internal decay. This disease has occurred in Maricopa County navel orange groves since the crop was first grown
in the region. Recently, however, the incidence and severity of Alternaria fruit rot in navel oranges has increased,
with annual losses as high as 0.5 box of fruit per tree. In citrus groves, Alternaria citri grows on dead citrus tissue
during wet weather, producing airborne spores. Initially, the fungus is believed to establish a quiescent infection
on the developing fruit. Colonization of fruit is facilitated when splits or growth cracks occur. The effectiveness of
fungicide treatments for disease control in the grove is uncertain, although certain fungicides are used to control
some diseases caused by other species ofAlternaria on other agricultural crops.
I The authors wish to thank the Arizona Citrus Research Council for supporting this project. This is the final
report for project 94-03 `Development of Control Measures for Alternaria Fruit Rot on Oranges in Arizona'
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The specific objectives of this research were to test the efficacy of two fungicides, Kocide 101 and Rovral
(iprodione), for disease control and to determine an application schedule that will result in optimum disease
control. The achievement of these objectives could provide the means to manage Alternaria fruit rot in Arizona
citrus groves.
Materials and Methods
Two field trials were established in navel orange groves with a high incidence of Alternaria fruit rot. In the 1994
study, a series of Rovral applications were sprayed onto test trees, beginning in April and continuing at one -month
intervals through August. Six groups of five 2 -tree replicates were established within each test citrus grove. One
group served as an untreated control. Five of the groups were treated with Rovral in April, four groups in May,
three groups in June, two groups in July, and one group in August. This sequential treatment scheme was
established to help identify the critical time period when infection occurs and when fungicide treatment is
necessary. An additional treatment consisted of a monthly Kocide application from March through August, 1994.
In the 1995 trial, Rovral applications were started in March and terminated in August, so that treatments consisted
of one to six applications of this fungicide. Kocide treatments were applied once in November or December in
1994 or January, February, or March in 1995. Disease development was monitored by determining the amount of
fruit drop occurring before harvest as well as final disease severity and yield loss at crop harvest.
Results and Discussion
In 1994 a significant reduction in the number of dropped oranges occurred on trees sprayed one to five times with
Rovral compared to nontreated trees (Table 1). Monthly applications of Kocide 101 from April through August
were no better than leaving trees untreated. In 1995, Rovral treatments from March through August provided no
apparent beneficial effects on control of Altemaria fruit rot, while a single application of Kocide 101 in December,
January, or February resulted in significantly less dropped oranges compared to nontreated trees (Table 2).
Because of the inconsistent activity of Kocide 101 and Rovral in these two studies, an additional trial is in progress
for 1996. The additional data from 1996 may help identify when and what fungicide(s) could provide significant
control of Alternaria fruit rot of navel oranges in Arizona.
References
Biggs, A.R., Ingle, M., and Solihati, W.D. 1993. Control of Alternaria infection of fruit of apple cultivar Nittany
with calcium chloride and fungicides. Plant Dis. 77:976 -980.
Brown, G.E., and Eckert, J.W. 1988. Alternaria rot. Pages 30 -31 in: Compendium of Citrus Diseases, ed. by
J.O. Whiteside, S.M. Garnsey, and L.W. Timmer. APS Press. St. Paul, MN. 80 pp.
DeWolfe, T.A., Erickson, L.C., and Brannamen, B.L. 1959. Retardation of Alternaria rot in stored lemons with
2,4 -D. Proc. Am. Soc. Hortic. Sci. 74:367 -371.
Hine, R., Matheron, M., and True, L. 1993. Diseases of citrus in Arizona. Ariz. Coop. Ext. Publ. 192038, 8 pp.
Pelser, P. du T. 1974. Influence of thiabendazole and benomyl on Alternaria rot in stored grapefruit. Citrus
Subtrop. Fruit J. 486:15 -17.
Schiffmann- Nadel, M., Waks, J., Gutter, Y., and ChaIutz, E. 1981. Alternaria rot of citrus fruit. Proc. Int. Soc.
Citriculture 2:791 -793.
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Table 1. Maricopa County 1994 Alternaria fruit rot fungicide trial
Treatment Month(s) Total number of dropped
oranges per tree
Untreated control 114 b'
Rovral April 77 a
Rovral April - - >May 72 a
Rovral April- ->June 72 a
Rovral April- ->July 66 a
Rovral April -- >August 71 a
Kocide 101 April -- >August 96 b
Numbers followed by a different letter are significantly
different (P= 0.05).
Table 2. Maricopa County 1995 Alternaria fruit rot fungicide trial
Treatment Month(s) Total number of dropped
oranges per tree
Untreated control 105 cl
Kocide 101 November 88 c
Kocide 101 December 62 a
Kocide 101 January 53 a
Kocide 101 February 66 ab
Kocide 101 March 84 be
Untreated control 64 ab
Rovral March 50 a
Rovral March --> April 69 be
Rovral March - ->May 64 ab
Rovral March -- >June 83 cd
Rovral March- ->July 97 d
Rovral March -- >August 72 be
For each fungicide and its respective untreated control,
numbers followed by a different letter are significantly
different (P= 0.05).
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