Baujard, Pierre

Duncan, Larry

Horizon / Pleins textes

Influence of nematicide dacement  depth  and 
time  of application onl treatment .efficacy 
in  the  Sahelian zone of Senegal 
Larry W. DUNCAN* and Pierre BAUJARD 
Laboratoire de Nématologie, ORSTOM, B.P. 1386, Dakar, Sénégal. 
SUMMARY 
Four doses of DBCP were  applied in  field  trials  1,8, 15, and 22 days  following the  first  seasonal  rainfall  of  1984 in the Sahelian 
Zone  of  Senegal.  Application  date  had no effect on postharvest  population  levels  of Scutellonema cavenessi when  doses  were  greater 
than  the  recommended  rate of 22.5 kg/ha  a.  i.  DBCP. An inverse  relationship  between  mid-season  population  levels  of S. cavenessi 
and  treatment  date was  observed at  the low rate of  11.25  kg/ha  i.  e.  DBCP.  Although  fewer than 1 O/o of S. cavenessi survived in 
fifteen of the sixteen  treatments,  peanut  plant  weights  were  positively  correlated  with  DBCP  dose  (r = 0.32*) when data were 
blocked  by treatment  date  and expressed  as  arcsin  of  the  proportion  of  each  week’s  highest  yield.  DBCP  and  metam  sodium  reduced 
population  levels  of S. cavenessi in  a log. linear  fashion  at  two  experimental  locations  when  application  depths  increased  from 5 
to 15 cm. 
RESUME 
Influence de  la profondeur et du  moment d’application de nématicides sur l’efficacité  du  traitement, 
dans la zone sahel’ienne du Sénégal. 
Lors  de  traitements  en  champ  dans  la  zone  sahélienne du Sénégal,  quatre  doses  de DBCP  ont  été  appliquées  1,8,  15  et 22 jours 
après  la  première (( pluie  utile D. Le moment du traitement n’a  pas  d’influence sur les  niveaux  des  populations  de Scutellonema 
cavenessi demeurant  après la récolte  si  les  doses  utilisées sont  plus  fortes  que  celle  usuellement  recommandée (22,5 kg/ha  m.  a. 
DBCP).  Une  relation  inverse  entre  les  niveaux  des  populations  de S. cavenessi à mi-culture et le  moment du traitement  a  été  observée 
pour la dose faible (11,25 kg/ha m. a., DBCP). Bien que moins de 1 O/O des S. cavenessi aient survécu dans quinze des seize 
traitements,  les  poids  des  arachides  sont  en  rapport  direct  avec  la  dose  de  DBCP  (r = 0.32)  si  les  données  sont  groupées  par  date 
de  traitements  et  exprimées  en  arcsinus  de  la  proportion  de  la  plus  forte  rkcolte  de  chaque  semaine.  Le  DBCP  et  le  métam  sodium 
réduisent  les  niveaux  des  populations  de S. cavenessi suivant  une  loi  log.  linéaire  dans  deux  des  sites  expérimentaux  lorsque  la 
profondeur  d’application  croit  de 5 à 15  cm. 
Nematicide and pathogenicity trials during  four 
growing  seasons in the Sahelian Zone of Senegal have 
demonstrated that soil fumigation may  be profitable for 
small plot farmers of peanut, millet, and  sorghum 
(Germani & Gautreau, 1976; Germani, 1979,  1981). 
Horse-drawn planters, modified to permit  simultaneous 
soil fumigation, are commercially available in Senegal. 
In spite of favorable field performance (Havard, 1984), 
the  utility  of these  machines is limited by the manner in 
which they are currently used. Fumigant application 
depth of 15 cm gave excellent control of al1 phytopara- 
sitic  nematodes in previous work.  However, horses are 
unable to pull the machines for longer than 3 hr/day, 
primarily due to the injection shank drag force. It is 
estimated that reducing the shank depth from 15 to 
10 cm reduces the effort  needed to pull the machines by 
nearly half (Havard, 1984). 
Another limitation to nematicide use is imposed by 
rainfall distribution in Senegal. Precipitation is unimo- 
dally distributed, occuring between May  and  October 
with drought conditions during the winter  months. 
During the drought period, soil moisture falls to less 
than 0.5 O/o of soil weight and nematodes exist in an anhy- 
drobiotic state (Demeure, 1980). Thus, soil and biotic 
conditions are  unsuitable  for  fumigation  except during 
the rainy season. Presently, nematicide  applications 
occur at sowing for several reasons. First, due  to limited 
rainfall(lO0-400 mm/year), crops  are sown with the  first 
* Present  address : Citrus Research and  Education Center, 700 Experiment  Station  Road,  Lake Afied,  FL338.50, USA. 
Revue Nématoi.; 9 (2) : 135-139 (1986) 135 
L. W Duncan & P. BazLjard 
seasonal rain  to  maximize yields. This practice permits 
no  time  between  fumigation  and sowing; nevertheless, 
fumigation at sowing is possible since low doses of 
dibromo-chloropropane  or  ethylene  dibromide  are  effi- 
cacious and non-phytotoxic to germinating peanuts. 
Fumigation following harvest has not been attempted 
because  field  observations and laboratory  studies  indi- 
cate Scutellonema cavenessi Sher, 1964, the most im- 
portant  nematode  parasite of the region, is most sensi- 
tive to nematicides during the first 72 hr following 
rehydratation from  the  annual  drought period  (Germani 
& Reversat, 1982, 1983). Recommended  fumigation 
time is thus restricted to the first two to three days 
following the  first  seasonal  rainfall.  This  fact,  coupled 
with the large force required to draw the nematicide 
shank  through  the soil,  severely limits the annual  surface 
area which can be treated  by  horse-drawn  applicators. 
The cost of these machines to small plot farmers is 
relatively expensive and expanding  their  annual utility, 
by reducing drag or by extending the recommended 
treatment period, will enhance  their  value. 
This paper presents results of field experiments to 
measure the efficacy of  soil fumigation with two nemati- 
cides at three application depths and on the effect of 
treatment  date,  measured  from  the  first seasonal rainfall, 
on efficacy of soil fumigation  with  DBCP. 
Material  and  methods 
APPLICATION DEITH 
Two experiments were conducted to compare efficacy 
of dibromo-chloropropane  (DBCP, 22.5 kg/ha a.  i.) and 
metam sodium (38.25 kg/ha a. i.) applied at  depths of 
5,10  and 15 cm. Bath nematicides were diluted in water 
and  applied  at  rates of 100 1 mixture/ha. The fumigants 
were applied with Shell fumigun injectors in the first 
experiment with injection  points at 10 cm intervals on 
lines 40  cm  apart.  Plots were 3 m x 3 m with 3 m alleys. 
The six treatments  and  an  untreated  control were 
replicated six times in a completely random design. 
Treatments  occurred  6  May 1984, 24 hr following the 
first  annual  rainfall at  Darou Sale. This date was  well in 
advance of the  usual rainy season and plots were not 
sown with peanuts (cv.  55-437) until 21 days later. 
Nematode soil populations were sampled on 12 May 
and  16  October  by  combining twelve, 2.5 x 30 cm soil 
cores/plot into samples which were mixed and from 
which 250 cm3  subsamples were processed (Seinhorst, 
1962). Endoparasite  populations were sampled on 
27 July  by  extracting  nematodes  from  three  root 
systemslplot  (Seinhorst, 1950). 
In the  second  experiment,  the  same  treatments were 
applied with a  horse-drawn  applicator-planter at  Nebe 
on  16  June, 48 hr following the  first seasonal rainfall. 
Plots were 3 x 5 m and distances between treatment 
136 
lines alternated between 45 and 54 cm because the 
injection shank occurs on one side of the machine's 
center  line.  Peanuts were planted by hand  the following 
day. Root populations were sampled on 7  August  and 
soil  populations were sampled  12 November  and proces- 
sed as above. The soil types at  Darou Sale and Nebe 
were sand  and loamy sand, respectively. 
TREATMENT DATE 
The experiment was conducted on a  loamy  sand soil 
at Nebe.  Plots 3 m x 3 m  separated by 3 m alleys  were 
treated 24 hr, 8, 15, and 22 days following the first 
rainfall of 1984 (14 June)  to  observe  whether  treatment 
efficacy  varied  with  treatment  date.  Doses f 11.25,22.5, 
45, and 90  kg/ha a.  i.  of DBCP were applied on each date 
to identify a suitable dose in the eventuality that S. 
cavenessi became less sensitive to nematicide  effects as 
a function of time following the first rainfall. Treat- 
ments were applied with Shell fumiguns 15 cm deep 
each  30 cm  on lines 30  cm  apart.  Nematicide doses were 
mixed with water and applied at a rate of 500 1 mix- 
ture/ha. Treatments were replicated three times in a 
randomized  complete block design.  Peanuts (cv.  55-437) 
were planted immediately following treatment each week. 
To simulate standard field practices, plots were not 
cultivated  for weed control  prior to  treatment  and 
sowing. Weeds were removed by hand  at  intervals  fol- 
lowing peanut emergence. Nematode population levels 
were estimated  from  samples  obtained  prior to the  first 
rainfall by combining six subsamples/plot taken to a 
depth of 15 cm, and samples were subsampled and 
extracted as previously described.  Nematode popu- 
lations in peanut roots were estimated 50 days following 
sowing  and  final  populations were estimated  from soil 
samples taken seven days following harvest. Peanuts 
were  harvested 90 days after sowing. Haulms  and seeds 
were dried and weighed. Yield data were blocked by 
treatment week, due  to variable rainfall, and expressed 
as proportions of each week's highest yielding plot. 
Transformed  (arcsin) yield data were regressed  against 
nematicide dose. 
Results 
APPLICATION DEFTH 
Root populations of S. cavenessi measured 50 days 
after  sowing were greater  at the 5 cm  depth  than  at 10 
or i5 cm depths at Darou Saie Îor both nematicides 
(Tab. 1). No consistent  reatment  differences were 
detected with 50 day root population levels at Nebe. 
Post-harvest soil population levels of S. cavenessi were 
negatively  correlated with placement depth  for  DBCP 
at  both locations and  for metam  sodium  at  Nebe  (Fig. 1). 
At  Darou Sale, metam sodium  only  reduced  population 
Revue Nématol., 9 (2) : 135-139 (1986) 
I?z$uence of depth  and  time  on treatnzent eficacy 
ol-..,--. . , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 
4 6 a 10 12 14 16 
Nematicide application depth ( C d  
Fig. 1. Postharvest  population  levels  (In)  of Scutellonema 
cavenessi following soil fumigation at different depths with 
metam  sodium  (squares)  or  DBCP  (diamonds  and  triangles). 
Table 1 
Mean  population  levels of Scutellonema cavenessJ100 g 
peanut  roots 50 days  following  sowing. 
Numbers  in  same  group  followed  by  different  letters 
are  significantly  different (p = 0.05) 
according to Duncan’s  multiple  range  test 
on ln transformed  data. 
Nematicide  Depth  Location 
(cm) Darou  Sale  Nebe
Metam  sodium 5 6 741 a 9  498 a 
10 3 837 a 8 056 a 
15 3  562 a 3  274 a 
DBCP 5 946 a 697 a 
10 52 b 282 a 
15 100 b 958 a 
levels with  respect to control  plots (average final  popu- 
lation = 1,905/1 soil) by 25, 48, and 35 O/o at depths of 5, 
10, and 15 cm, respectively. By contrast, DBCP placed 
at the 15 cm depth at Darou Sale reduced final soil 
populations of S. cavenessi by more than 99 O/o with 
respect to control  plots. The degree of placement depth 
effect, as measured by the slope of regression  lines  fitted 
to log  transformed  data, was the  same at both  locations 
where DBCP was used (Fig. 1). The sampling  strategy 
for soil  populations was more  precise than  that  for  root 
populations; average coefficients of variation were 81 
and 162 (p = 0.05), respectively. 
Revue  Neinatol., 9 (2) : 135-139  (1986) 
TREATMENT DATE
There were no  effects of treatment  date  on  root  or soil 
population levels of S. cavenessi when doses of DBCP 
were greater  than 11.25 kg/ha  a. i. (Tab. 2, Fig. 2). An 
inverse relationship between treatment date following 
the  first seasonal rainfall  and  root  populations 50 days 
after sowing existed when the low rate of  11.25 kglha a. 
i.  was used (Fig. 3). This relationship was not  noted  in 
the post-harvest soil population levels although final 
populations in plots  treated  on the day  following the  first 
seasonal  rainfall were higher than those treated on 
subseauent dates for the 11.25 kg/ha a. i. treatment 
(Fig. 3. 
l D O l  
71.25 
1 
1 2 3  
22.5 
i 
3 4  1 
45 
treatment ueek 
90 
dose (kglha a.i.> 
Fig. 2. Effect of treatment  date on reduction of postharvest 
Scutellonema cavenessi soil population levels from preplant 
population  levels. 
Table 2 
Population  levels  of Scutellonenza cavenessiIIO0 g 
peanut  roots 50 days  following  sowing. 
Mean  values  followed  by  standard  errors (n = 3). 
Days 
following 
first  rainfall Dose (DBCP k g h a  a. i.) 
11.25  22.5  45 90 
1 827  (577) 21 (21) O O 
8  135 ( 45) O 19 (19) O 
15 44 ( 44) O 36 (36) O 
22 O O O 63  (63) 
137 
L. W: Duncan & P. Baujard 
Plant  growth  declined  with  treatment-planting  date. 
The linear correlation coefficient between top growth 
and  planting  date (days following the first  rainfall) was 
- 0.82** and between seed yield and planting date  it 
was - 0.76**. A positive linear  correlation  (arcsin 
y = 0.66 + 0.005 dose  DBCP,  r = 0.32") existed 
between 90 days peanut vegetative  growth  and  dose of 
DBCP when data were blocked according to planting 
date  and expressed  as  proportion of the greatest growth 
for  a given date. 
Discussion 
The results obtained suggest that during the rainy 
season in Senegal the efficacy of DBCP applied at  the 
recommended rate of 15 l/ha (22.5 kg/ha a. i.) is not 
influenced by treatment  date (Fig. 2). In contrast, 
reduction of the recommended nematicide placement 
depth (15 cm) is likely to reduce the  product efficacy 
(Fig. 1). 
The hypothesis that treatment efficacy of 15 l/ha 
DBCP is inversely related to time between the first 
seasonal  r infall and nematicide  tr atment  date 
(Germani & Reversat, 1983) was based  on field obser- 
vations  (Germani & Reversat,  1982)  and on laboratory 
experiments (Germani & Reversat, 1983) that clearly 
demonstrated S. cavenessi is  more  susceptible to  DBCP 
toxicity during a  short  period  following  anhydrobiosis. 
Nevertheless,  data  from  those  experiments  appear to  be 
consistent  with the  present results.  Germani  and  Rever- 
sat (1983) noted  that,  regardless of whether  nematodes 
were treated immediately or twelve days following re- 
hydratation, mortaliq increased  with t h e  following 
DBCP  treatment, a  frequent  -field  observation (G. 
Germani, P. Baujard & L. Duncan,  unpublished  data), 
and that nematodes surviving treatment were repro- 
ductively  sterile.  Nematode  population levels were 
monitored  for  fourteen days following  treatment in  that 
study  compared  with 50 and 97 days in  the  present work. 
Thus, while DBCP is most  toxic to S. cavenessi during 
rehydration from the anhydrobiotic state, there is no 
evidence that 15 l/ha  DBCP in  the field will not eventu- 
ally reduce S. cavenessi populations to non-detectable 
levels (Fig. 2), regardless of treatment  date  during  the 
rainy season. The present  data  suggest  that  under  field 
conditions,  nematode  survivorship  may actually be  en- 
hanced by early treatment  dates when low doses 
(1 1.25  kg/ha.a.i.) of DBCP are  used (Fig. 3). Since weeds 
were not cultivated in  treatment  plots  until  the  date of 
nematicide  treztment, it is possible that riematodes 
which established  themselves in weed roots were killed 
when  roots were removed  from the soi1 during  the  later 
treatment  dates.  If  the S. cavenessi mortality  rate is Ca. 
98 O/o when 11.25 kg/ha a.  i. DBCP is used  under  these 
conditions, weed cultivation  may  explain the additional 
mortality at later  dates. 
138 
P l  
O 
O 2 4 6 8 10 12 14 16 18 20 22 
DaYa follouing f i r a t  annual rainfall 
Fig.  3.  Effecr  of  treatment  date on  population levels  of Scutel- 
lonema cavenessi in peanut  roots  50  days  after  sowing. 
Finally,  numerous  studies  (Germani & Gautreau, 
1976; Baujard, Duncan & Germani, 1982; Baujard et al., 
1985)  indicate the unlikelihood that  15 l/ha DBCP may 
be  sufficient to eradicate  populations of S. cavenessi in 
conditons other than carefully controlled small plot 
experiments. This fact  mitigates the  importance of slight 
effects  that variables such as treatment  date may have 
on  treatment efficacy. 
There are few if any reports that  DBCP  augments 
plant growth by means  other than  nematode suppres- 
sion. The positive correlation in the treatment date 
experiment between peanut plant weights and  DBCP 
dose  is  surprising  since  phytoparasitic  nematodes were 
reduced  to  extremely low or non-detectable levels in al1 
replicates of  al1 treatments.  Recent  investigations (Dun- 
can, 1985) suggest that S. cavenessi is considerably  more 
pathogenic on millet  and cowpea than  on peanut.  If the 
present results are supported by fùrther research, they 
would help explain growth augmentation in peanuts 
following  treatment by DBCP  in  terms  other  than mere 
suppression of nematode  populations. 
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Accepté  pour  publication le 20 décembre 1985. 
139 


Influence of nematicide placement depth and time of application on treatment efficacy in the Sahelian zone of Senegal

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