(0- \ AGRICULTURAL RESEARCH CENTER
\ Hastings, Florida
Mimeo Report PR-1972-1 September 1972
POTATO LATE BLIGHT CONTROL IN NORTHERN FLORIDA
D e L R lant Pathologist
NOV 16 197
The most severe out reak of potato late bli t since 1965 occurred in North
Florida during 1972 Although the disease ccurs in the area each year, epiphy-
totics have been un pgmAn during the past 2 years. Use of an effective late
blight forecast sysi f i ride9a QfiIrtda resistant varieties, and availability
of effective fungicides have usu ly ll e disease in check. The late blight
outbreak in 1972 was due to several factors. First, inoculum was introduced into
the area in infested seed lots. Secondly, weather conditions were conducive to
late blight spread during the entire season. Thirdly, many growers were forced
to abandon early season spray applications due to delays in planting schedules
brought about by heavy rains. In addition, much of the early, nonsprayed potato
crop consisted of highly susceptible varieties such as Red LaSoda. Once late
blight became widely established in early plantings, spread of the disease was
imminent even in the resistant Sebago variety.
Effective late blight control in northern Florida is dependent upon several
factors: (1) Use of certified seed; (2) Late blight resistant varieties
should be utilized as much as possible; (3) Growers must heed late blight warn-
ings as they are issued; (4) Effective fungicides must be applied at the correct
time, in effective dosages, and in an effective manner.
Seed Certification and Inspection. All available evidence indicates that late
blight inoculum (spores) is introduced into North Florida in infected seed.
There is no scientific proof that late blight spores blow into North Florida
from infested fields in the south. We should however, be alerted to late blight
during years when it is damaging in South Florida because weather conditions favor-
ing the disease can often occur in both sections of the state during the same
season. These factors being considered, use of certified seed meeting tolerance
for late blight accompanied with local inspection and careful grading of seed
during cutting can help reduce the amount of inoculum being introduced into the
Resistant Varieties. Most newer varieties of potato possess resistance to one or
more strains of the late blight fungus. Sebago has good field resistance. Other
late blight resistant varieties suitable for the Hastings area include: Pungo,
Wauseon, LaChipper,. and Penn 71. Red LaSoda and Norchip are not resistant to the
common late blight fungus.
Forecast System. Detailed weather data are compiled at the ARC, Hastings. When
weather conditions are favorable for late blight development, a forecast is issued
for the area via newspapers and Extension Newsletters. Although a more sophisti-
cated forecast system is being investigated, our existing system has usually
S proven reliable during years of severe late blight outbreaks. Growers should
Iatch for early warnings and initiate spray schedules as such warnings are issued.
Effective Fungicide Program. a) Effective application Experimentation is being
done in other sections of the U. S. with application of fungicides in low gallon-
ages of water, however, until such data are available for Florida, fungicides
should be applied in 100-150 gallons water/acre delivered at 100-250 psi via six
Aerial application of fungicides has been effective against potato late blight
in northern states and in South Florida. No data on effectiveness of aerial
application of fungicides under a late. blight epiphytotic are available for the
Hastings area. However, many growers apparently were satisfied with aerially
applied fungicides during the 1972 season. When fungicides are applied from the
air, just as with ground application, proper timing, thorough coverage of the
field, proper nozzles, and adequate dosage are vital. Aerial application has not
been formally recommended for general use by the University of Florida Extension
b) Effective fungicides Two late blight control experiments were performed at
ARC, Hastings in 1972. The results of these tests are reported below.
All fungicides were evaluated on Red LaSoda potatoes. Chemicals were applied in
75 gallons water/acre via 4 nozzles/row at 100-150 psi. Materials were applied in
Test 1 on 4/6-7, 4/10, 4/17, 4/25, and 5/5; andin Test 2 on 4/5, 4/7, 4/17, 4/25,
and 5/5. Late blight was first observed in the test plots 4/4/72 and the disease
became epiphytotic in nontreated plots by the first week in May. In addition to
late blight control, both experiments were designed to determine whether applica-
tions of copper fungicides would reduce skinning of potato tubers during harvest.
Results are shown in Tables 1 and 2. Each of the currently recommended fungicides
(Polyram 80W, Manzate D 80W, Dithane M-45 80W, and Bravo 75W) and Difolatan 4F
effectively controlled the disease. Fungicides containing copper were not as
effective but provided some control under conditions of the tests. None of the
materials tested reduced skinning of harvested tubers.
Late blight on potatoes can be effectively controlled in northern Florida through
combined use of (1) Seed certification and local inspection; (2) Potato
varieties resistant to late blight; and, (4) Properly applied, effective fungi-
cides. Fungicides currently suggested for grower use by the University of Florida
Extension Service include: Maneb (Manzate D and Dithane M-22 Special) 80% 1.5
Ib; Dithane M-45 80% 1.5 Ib; Manzate 200 80% 1.5 lb; Polyram 80% 1.5 lb;. and
Bravo 75%. 1.5 lb..
PR-1972-1 Page 2
Table 1. Summary of 1972 late blight control experiment ARC Hastings, Florida (Test l)-/
Disease Index2/ Skin3/ Tubers Specific
Treatment 4/28 5/01 5/08 5/09 Index (Cwt/A) Gravity
Bravo 75W 1.5 lb. 1.25 a 1.25 a 2.3 a 2.0 a 5.9 241.1 a .1.051 a
Bravo 6F .75 qt. 1.25 a 1.00 a 2.3 a 2.3 ab 6.5 234.6 ab 1.049 ab
Difolatan 4F 1.0 qt. 1.50 a 1.25 a 2.5 a 4.3 de 6.0 223.5 ab 1.048 bc
Polyram 80W 1.5 lb. 1.25 a 1.00 a 3.8 ab 3.8 bcde 6.0 215.0 ab 1.047 bc
Kocide 101 86W 1.75 a 2.00 bc 6.0 c 6.8 f 7.3 213.7 ab 1.046 c
Thynon 75W 1.5 lb. 1.75 a 2.25 c 3.8 ab 4.8 e 6.5 212.4 ab 1.049 ab
Copper Count N 3 qt. 1.00 a 1.50 ab 4.5 bc 5.5 f 7.5 206.5 abc 1.046 c
Manzate D 80W 1.5 lb. 1.25 a 1.25 a 3.3 ab 4.0 cde 6.1 204.5 abc 1.051 a
Cit-Cop 4E 1.0 qt. 1.75 a 3.25 d 7.8 d 8.8 g 7.4 200.6 bc 1.048 bc
Control ... ....5.50 b 7.25 e 9.5 e 11.0 h 5.3 172.5 c 1.046 c
LSD 1% 1.5 0.4 2.4 2.2 NS NS NS
LSD 5% 2.0 0.6... 1.8 1.6 NS 32.0 0.003
1/ ll1 data represent mean values of 4 replications. Column values followed by same letter are nonsigni-
ficant at 5% via Duncan's multiple range test. Lack of letters denotes nonsignificance.
2/ Disease index (1-11). 1 = no plants infected; 1 = all plants severely infected.
3/ Skin index (1-10). 1 = no skinning; 10 = totally skinned.
PR-1972-1 Page 3
Table 2. Summary of 1972 late blight control experiment at ARC Hastings, Florida (Test 2)1/
3/ 4/ Marketable
Disease Index" Skin- Tubers Specific
Treatment2/ 4/28 5/01 5/08 5/09 Index (Cwt/A) Gravity
Kocide 2.0 (1st 3), Dithane 1.5 (last 2) 2.4 a 2.6 a 4.4 a 6.0 ab 5.2 183.0 1.046
Dithane 1.5 1.0 a 2.0 a 3.6 a 5.2 ab 5.6 200.0 1.048
Dithene 1.5 (1st 3), Kocide 2.0 (last 2) 1.8 a 1.8 a 3.6 a 4.4 a 5.1 197.4 1.047
Dithane 1.5 + Kocide 1.0 2.2 a 2.6 a 3.4 a 4.6 ab 5.0 175.8 1.050
Dithane 1.5 + Kocide 2.0 1.6 a 2.6 a 3.4 a 5.4 ab 4.5 187.6 1.051
Kocide 1.0 2.4 a 2.8 a 4.8 a 6.8 b 5.8 173.2 1.046
Kocide 2.0 2.2 a 2.4 a 4.0 a 5.4 ab 5.7 184.9 1.048
Control 5.6 b 7.0 b 9.0 b 10.0 c 4.4 153.6 1.046
LSD .01 2.8 2.5 2.1 2.8 NS NS NS
LSD .05 2.1 1.9 1.5 2.1 NS NS NS
1/ All data represent mean values of 5 replications. Column values followed by same letter are nonsignificant
at 5% via Duncan's multiple range test. Lack of letters denotes nonsignificance.
2/ Fungicide formulations used were: Kocide 101 861 and Dithane M-45 80W.
3/ Disease index (1-11). 1 = no plants infected; 11 = all plants severely infected.
4/ Skin index (1-10). 1 =-no skinning; 10 = totally skinned.
PR-1972-1 Pare 4