Two studies have recently been published, comparing subsurface drip and furrow irrigation as they affect diseases of lettuce. The lettuce varieties used in the study were Salinas and Misty Day. Conventional furrow irrigation was compared to irrigation provided by 0.5 mm T-tape with 8-inch emitters buried at 10 inches from the bed top. The lettuce was thinned to 12 inches. The test area was inoculated with Sclerotinia minor (lettuce drop). Data were also collected on the incidence of corky root (Rhizomonas suberifaciens) and downy mildew (Bremia lactucae) as well as the incidence of lettuce drop. The research reported was done on a spring and fall crop each year during 1993, 1994 and 1995 in the Salinas Valley.
Lettuce drop incidence was not different between the two treatments during the first two weeks after the irrigation treatments began. However, the treatments then began to differentiate and, consistently, the incidence of lettuce drop was significantly lower in the subsurface drip-irrigated plots at harvest time. Lettuce drop incidence was consistently 12 to 25% less in the buried drip plots compared to the furrow-irrigated treatments both in the spring and in the fall trials. Misty Day cultivar lettuce has corky root resistance, therefore, corky root disease ratings were only significant on the spring trials where the cultivar Salinas was used. Observations in these trials showed that buried drip-irrigated plots had significantly lower disease incidence. Subsurface drip irrigation alone reduced corky root severity by more than 50% in two of the three trials and by nearly 50% in the third trial.
Downy mildew was not affected by the method of irrigation. The incidence of this disease was similar in both irrigation treatments.
Lettuce yield and marketable heads were also influenced by irrigation method. Proportionally, the number of harvestable plants was significantly higher under subsurface drip irrigation, as was the number of marketable heads, and the overall yield during each season regardless of the cultivar. The above information is presented in Table 1.
Table 1. Lettuce drop incidence at harvest, number of marketable heads, proportion and volume of marketable heads, total yield, and corky root severity on two lettuce cultivars in subsurface drip- and furrow-irrigated plots during the spring and fall lettuce crop seasons of 1993 to 1995 in the Salinas Valley, CA.
| Year and Season | Cultivar | Irrigation | Drops (%) | NMHt | Proportion of Headsu | Volume of Heads (cm3)v | Yield (kg)w | CRx |
| 1993 Spring | Salinas | Drip | 21.6 by | 28.8 a | 67.2 a | 969 a | 15.7 a | 3.8 b |
| Furrow | 41.8 a | 21.2 b | 52.1 a | 580 b | 9.5 b | 8.9 a | ||
| 1993 Fall | Misty Day | Drip | 31.4 b | 22.3 a | 53.1 a | 940 a | 10.1 a | 3.8 a |
| Furrow | 59.8 a | 13.6 b | 32.2 b | 948 a | 6.7 b | 4.5 a | ||
| 1994 Spring | Salinas | Drip | 3.3 b | 38.3 a | 70.2 a | 802 a | 21.9 a | 3.3 b |
| Furrow | 33.8 a | 24.3 b | 42.2 b | 680 b | 14.1 b | 9.4 a | ||
| 1994 Fall | Misty Day | Drip | --z | -- | -- | -- | -- | -- |
| Furrow | -- | -- | -- | -- | -- | -- | ||
| 1995 Spring | Salinas | Drip | 10.3 b | 42.3 a | 85.3 a | 1,516 a | 31.0 a | 4.4 b |
| Furrow | 27.4 a | 37.8 b | 73.5 b | 1,011 b | 25.0 b | 7.2 a | ||
| 1995 Fall | Misty Day | Drip | 15.1 b | 19.0 a | 48.5 a | 1,218 a | 9.4 a | 2.6 b |
| Furrow | 41.8 a | 8.7 b | 24.8 b | 1,167 a | 4.1 b | 3.1 a |
s Mean lettuce drop incidence in the central 24-m2 area of the middle four beds of each treatment plot.
t Mean number of marketable lettuce heads in the central 24-m2 area of the middle four beds of each treatment plot.
u Mean of the ratio of the number of marketable heads to the total number of lettuce plants in the 24-m2 area.
v Mean volume of lettuce heads.
w Mean weight of the total marketable heads.
x Mean corky root severity estimated by the scale of O'Brien and van Bruggen (17).
y Within each column and season, numbers followed by the same letter are not significantly (P> 0.05) different according to an LSD test.
z Crop not harvested.
In this trial, subsurface drip irrigation had a positive effect on yield and head quality while suppressing lettuce drop and corky root diseases as compared to conventional furrow irrigation. This translates into more marketable heads per acre with the use of buried drip irrigation. Cost comparison figures (buried drip vs. furrow irrigation) were not presented as a part of this study.
In the second study the factors in drip irrigation that affect lettuce drop (Sclerotinia minor) were accessed. Buried drip and furrow irrigation were compared in terms of soil moisture distribution, soil temperature fluctuations, and soil microflora (actinomycetes, bacteria and fungi). The soil moisture patterns differed markedly between the two irrigation systems. The soil moisture under subsurface drip was significantly lower at all depths and distances from the bed center after an irrigation event than under furrow irrigation. The soil temperature was significantly higher at both 2- and 6-inch depth under drip irrigation as compared to furrow irrigation. There were no quantitative differences in the three groups of microflora between the two irrigation systems. Organisms were found that suppressed the growth of Sclerotinia sclerotia. However, these organisms were found in both irrigation regimes.
The second study concludes that suppression of lettuce drop under buried drip irrigation compared to furrow irrigation is attributed to differential moisture and temperature effects rather than to changes in the soil microflora or their inhibitory effects on S. minor.
Research conducted in Ohio on bacterial leafspot of lettuce, caused by Xanthomonas campestris pv. vitians, was recently reported in the Journal of Plant Diseases. The conclusions of the research were as follows:
Xanthomonas campestris pv. vitians is a seedborne pathogen. Treatment of infected seed with sodium hypochlorite (NaClO) at 0.52% reduced seed contamination from 10.5 to 0.03% without significantly affecting seed germination.
Lettuce varieties did show differences in their susceptibility to X.c.v. However, Darkland (romaine) was the only western-used variety tested, and it was found to be highly susceptible. No head lettuce varieties were tested.
None of the X.c.v. strains tested were found to show resistance to copper sulfate fungicides. And copper-containing fungicides were found useful in suppression of the disease under field conditions.
Lettuce should be rotated away from fields where the disease has occurred. Deep plowing to bury infected plant debris was also suggested.
Finally, avoid overhead (sprinkler) irrigation or stop its use as soon as bacterial leafspot is found in a field. Also, do not move equipment or hoeing crews through the field while plants are wet.
