Foliar Fungal - Alternaria Brown Spot - Alter-Rater

The ALTER-RATER is a weather-based point system to assist in the timing of fungicide applications for control of Alternaria brown spot. The system was developed based primarily on the amount of infection which occurred on susceptible trap plants placed in a heavily infested Minneola tangelo grove for 24-hour periods. Rainfall was an important factor affecting disease incidence, but the amount of rainfall was not highly significant as long as it was more than a trace. Days with leaf wetness durations of more than 10 hours had more disease than days with fewer numbers of hours. Alternaria infection was high over a wide range of temperatures but did diminish if the average daily temperature was above 83 or below 68°F.

To use the system, points are assigned to each day based on the rainfall, leaf wetness and average daily temperature as shown in Table l. The daily point scores are totaled and a fungicide application is made when the selected threshold value is reached. Threshold values need to vary with the susceptibility of the variety and the disease history in each grove. Suggested threshold values are given in Table 2. However, with experience in a particular grove, the threshold value can be adjusted to suit the situation.

In using this system, the first fungicide spray should be applied when the spring flush reaches 2-3 inches in length. Thereafter, all sprays should be based on the weather, that is, the accumulated point score. In the example in Table 3, if the first spray was applied on Mar. 14, then the second application would be applied after Mar. 28 if the 50-point threshold is being used or after Apr. 15 if the 100-point threshold is used. Each time a spray is applied the score returns to zero. The example in Table 4 assumes a spray was applied on May 14. Thereafter, a spray would be needed after May 23 if the threshold of 50 is used, after May 30 if the 100 threshold is used, but not until after June 11 if the 150 threshold is used. No sprays are recommended after July 15 by which time the fruit should be resistant to infection. Table 5 illustrates the number of sprays that would have been required using the ALTER-RATER in Lake Alfred, the Green Swamp and in Immokalee in different seasons as well as the intervals between sprays. Ft. Pierce is not illustrated but it was virtually the same as Immokalee.

Currently, copper fungicides, Abound, and Trilogy are registered and recommended for disease control. No more than 2 lb of metallic copper should be applied per acre per application even with average quality products. When applications are being made frequently and high quality copper fungicides are used, rates as low as 0.75 lb of metallic copper per acre should provide good control. Trilogy at 1% of the spray volume or Abound at 12-15 fl.oz/acre can be substituted for copper fungicides and is especially indicated in June and July. Copper products applied when temperatures are high can cause fruit injury and darken existing blemishes.

Relevant publications

1.Timmer, L.W. and A. Bhatia. 2001. Evaluation of the Alter-Rater model for timing fungicide applications for control of Alternaria Brown Spot of citrus. Phytopathology 91:S88-89.

2. Timmer, L. W. 2000. The ALTER-RATER, a new weather-based model for timing fungicide sprays for Alternaria control. Citrus Industry 81(3): 23-24.

Table 1. Daily Points Assigned

 

Rainfall > 0.01 inch

Leaf wetness > 10 hr

Average daily temperature

Assigned score

+

+

68-83

11

+

+

>83

8

+

+

<68

6

+

-

68-83

6

+

-

>83

4

+

-

<68

3

-

+

68-83

6

-

+

>83

6

-

+

<68

4

-

-

68-83

3

-

-

>83

0

-

-

<68

0

 


Table 2. Suggeated Threshold Score

 

Score

Situation

50

Heavily infected Minneola, Dancy, Orlando, Sunburst; many flatwoods groves, east coast and SW Florida

100

Moderately infested Minneola or Dancy, many Murcotts; Ridge and North Florida groves

150

Light infestations, any variety, mostly Ridge and North Florida groves

 


Table 3. Scoring Example

Date

Rain

LeafWetness>10h

Avg.Daily Temp

Daily Score

Cumulative Score

3/15

-

-

< 68

0

0

3/16

-

-

<68

0

0

3/17

-

+

68-83

6

6

3/18

-

+

68-83

6

12

3/19

-

+

68-83

6

18

3/20

-

+

68-83

6

24

3/21

-

-

<68

0

24

3/22

-

+

68-83

6

30

3/23

-

-

68-83

3

33

3/24

-

-

68-83

3

36

3/25

+

-

68-83

6

42

3/26

-

-

68-83

3

45

3/27

-

-

68-83

3

48

3/28

-

-

68-83

3

51 XX

3/29

-

-

68-83

3

54

3/30

-

-

68-83

3

57

3/31

-

-

< 68

0

57

4/1

-

-

< 68

0

57

4/2

-

-

< 68

0

57

4/3

-

-

< 68

0

57

4/4

-

-

< 68

0

57

4/5

-

-

68-83

3

60

4/6

-

-

68-83

3

63

4/7

+

+

< 68

6

69

4/8

-

-

68-83

3

72

4/9

-

-

< 68

0

72

4/10

-

-

68-83

3

75

4/11

+

+

68-83

11

86

4/12

-

-

68-83

3

89

4/13

-

-

68-83

3

92

4/14

+

+

< 68

6

98

4/15

-

-

68-83

3

101 XX

 


Table 5. Application of the Alternaria Model

 

Accumulated point score
Year
No.of applications (Minimum and Maximum spray intervals)
Lake Alfred
Green Swamp
Iimmokalee

50

1997

11 (7-15)

11 (9-17)

--

 

1998

8 (12-30)

10 (7-21)

--

 

1999

9 (11-22)

10 (8-26)

15 (5-14)

100

1997

6 (15-28)

6 (18-29)

--

 

1998

5 (19-45)

5 (22-38)

--

 

1999

5 (17-40)

5 (15-43)

8 (12-24)

150

1997

4 (31-43)

4 (26-44)

--

 

1998

3 (46-56)

4 (35-46)

--

 

1999

3 (40-54)

4 (25-55)

6 (18-31)

10-day program

--

13 (10)

--

--

14-day program

--

9 (14)

--

--

21-day program

--

6 (21)

--

--

Home

About Us

Academics

Research

Extension

Facilities

Publications

Resources

Services