Postinfection activity of selected Potato late blight fungicides
By Brad Geary, Matthew Sunseri, Tom Cummings and Dennis A. Johnson, Washington State University
Fungicide application to potato foliage is most effective in late blight management before infection
occurs since most fungicides have protective modes of action and are not effective after the fungus,
Phytophthora infestans, enters plant tissues. However, fungicides are not always applied in time: canopy
growth may be rapid, vast areas may need to be covered, custom applicators may need to be scheduled, or
weather may interfere. As a result, lesions in various stages of development may be present in a field
before fungicides are applied. Also, inadequate coverage of all foliage with fungicide may leave
unprotected patches. Therefore, a fungicide product that restricts disease progress once infection has
occurred may aid in slowing development of late blight and possibly reducing tuber infections. Little
information has been previously published on the effectiveness of currently used late blight fungicides in
inhibiting components of disease development after infection has already occurred.
RESEARCH METHODS
We had two objectives:
1. Evaluate the effectiveness of selected late blight fungicides in inhibiting lesion expansion and
sporulation (production of sporangia) after stem lesions had occurred on potted plants in the
greenhouse.
‘Russet Burbank’ potato plants were grown in the greenhouse and inoculated with the US 8 strain of P.
infestans. After lesions developed on the stems, the plants were sprayed until runoff with various
fungicides. The fungicides were Quadris (Azoxystrobin), (Bravo (chlorothalonil), Polyram plus Super
Tin (metiram plus triphenyltin hydroxide), Tattoo C (propamocarb hydrochloride plus chlorothalonil),
Curzate (cymoxanil) plus mancozeb), and Acrobat MZ (dimethomorph plus mancozeb). The plants
were sprayed again 5 to 7 days later. After each of the two fungicide applications, plants were placed
in a humid environment to allow sporulation on infected stems. Numbers of sporangia were then
visually described and counted and lesion length was measured.
2.
Evaluate the curative activity of selected fungicides 12 to 48 hours after detached potato leaflets were
inoculated with P. infestans.
‘Russet Burbank’ potato plants were grown in the greenhouse. Petioles with three leaflets were placed
in a nutrient solution in test tubes and each leaflet was inoculated with a drop of water containing
10,000 P. infestans sporangia/ml. At 12, 24, 36, and 48 hours following inoculation, leaflets were
sprayed until runoff with one of the four fungicides. Disease severity, incidence (frequency) of
infection, and numbers of sporangia produced were determined for all inoculated leaflets.
In both experiments, each fungicide was applied at a rate consistent with those used in commercial fields.
RESULTS
Lesions on control plants not sprayed with fungicide expanded an average distance of 0.8 to 1.7 inches
up stems from the inoculation point by the time that test plants received one fungicide application and
another 1.0 to 1.2 inches after test plants received the second fungicide application.
Objective 1.
Lesion expansion on stems was significantly restricted with each of the following:
 one or two applications of Acrobat MZ
 one or two applications of Polyram + Super Tin
 two applications of Tattoo C
Sporulation on stems was significantly reduced with each of the following:
 one or two applications of Acrobat MZ
 one or two applications of Curzate + mancozeb

two applications of Tattoo C
Objective 2.
Incidence of leaflet lesions was significantly reduced when:
 Tattoo C was applied 24 and 48 hours after inoculation
 Curzate + mancozeb was applied 12, 24, and 36 hours after inoculation
Lesion size (disease severity) on leaflets was significantly reduced when:
 Tattoo C was applied up to 48 hours after inoculation
 Curzate + mancozeb was applied up to 36 hours after inoculation
Sporulation on leaflets was significantly reduced when:
 Tattoo C was applied up to 48 hours after inoculation
 Curzate + mancozeb was applied up to 48 hours after inoculation
All three disease components were significantly reduced when:
 Acrobat MZ was applied 12 hours after inoculation but not later
SUMMARY/DISCUSSION
Lesion expansion and sporulation on infected stems was suppressed by one application of Acrobat MZ and
two applications of Tattoo C. Sporulation on infected stems was suppressed by one application of Acrobat
MZ, one application of Curzate + mancozeb and two applications of Tattoo C. Quadris and Bravo did not
suppress lesion expansion or sporulation. However, they and the other fungicides tested in this study
prevent new lesions when applied before infection
.
Late blight management is extremely difficult in the Columbia Basin once the disease is present in a
potato field. After row closure the humidity and temperature levels within the canopy usually favor
infection whenever sprinkler irrigation water is applied, which may be as frequently as every 1.5 to 3 days.
Fungicides that have post-infection activity, such as inhibition of sporulation and lesion expansion on
stems, may help suppress late blight outbreaks in fields, especially when disease levels are still relatively
low. Suppressing sporulation on potato foliage between tuber bulking and harvest is likely to decrease the
number of tuber infections. Restricting lesion expansion also may be beneficial in reducing tuber infection
since it would limit the total amount of plant tissue producing new spores.
This study showed that Tattoo C reduced lesion development (lesion number and size) and sporulation
on leaves when it was applied up to 48 hours after the leaves were inoculated with P. infestans. This time
period represents only a narrow window of opportunity to apply a fungicide after weather conditions favor
infection. An isolate of an aggressive strain, such as US-8, narrows the window even more. Because
growers generally cannot predict the exact time of infection, we recommend using the most effective
fungicide product with the broadest window of effectiveness.
Fungicides need to be integrated into a total disease management program that includes strict sanitation
practices, proper irrigation management, and cultural practices. However, late blight may appear in a field
before management practices are applied. Effective management practices in the Columbia Basin after late
blight is found in a field include: effective fungicides and application methods, shorter time intervals
between fungicide applications, fewer sprinkler irrigation applications, and irrigating when the crop canopy
will be wet for the least amount of time. Application of fungicides that suppress sporulation during and
after tuber bulking may reduce tuber infections. With regard to fungicides, however, the most economical
and effective timing for application is before infection.