Subsurface Drip Irrigation Reduces Alternaria Late Blight of Pistachio Caused by Alternaria alternata
(Second-Year Report)

Themis J. Michailides and David P. Morgan, U.C. Davis, Plant Pathology
and
David A. Goldhamer, U.C. Davis, Land, Air and Water Resources, Kearney Agricultural Center, 9240 5. Riverbend Ave., Parlier 93648

Abstract

The effects of subsurface drip and flood irrigation on Alternaria late blight of pistachio were compared in a 80-acre Kerman pistachio orchard for a second year. The experimental design was a complete randomized block design with five replications of 12 tree rows each of up to half a mile long. Similarly to the 1994 results, subsurface drip irrigation resulted in significantly lower incidence and severity of infected leaves by Alternaria alternata. For instance, by commercial harvest time only about 10% of the leaves in the blocks irrigated by subsurface drip were infected while 55% of those in the blocks irrigated by flooding were infected. Furthermore, subsurface irrigation reduced the incidence of infected fruit by half The levels of Alternaria (and other filamentous fungi and yeasts) spore inoculum on leaves and fruits were not affected by the irrigation neither the incipient infections by Alternaria on pistachio leaves. Subsurface drip irrigation resulted in shorter periods of dew formation per day, lower relative humidity, and higher temperatures. The differences in these environmental parameters can explain the differences m disease levels between the two irrigation systems. In general, subsurface drip irrigation did not reduce yields but resulted in significantly more split inshell nuts. In both systems of irrigation, fruit load on shoots predisposed leaves to infection by A. alternata. In conclusion, the results of the second year's study strongly suggest that management of Alternaria late blight in orchards irrigated by flooding can be successfully done by converting to subsurface drip irrigation.

Objectives

The objectives of tins study were similar to those of 1994:

1. to determine the effects of subsurface drip irrigation in altering the environmental conditions in the orchard in comparison with the flood irrigation,
2. to study the effects of the two systems of irrigation on Alternaria late blight disease (disease incidence and severity, shell staining, and contamination of nuts with molds), and
3. to determine the effects of the two systems of irrigation on the crop production parameters of pistachio. We also determined the effect of fruit load on disease susceptibility.

Background
Alternaria late blight, caused by Alternaria alternata (Fr.) Keissler; is a problematic disease of pistachios grown in California (see Annual Reports of the California Pistachio Commission for details on tins disease) (Michailides & Morgan, 1991a & 1991b, and 1992). The pathogen infects both leaves and fruit and causes early defoliation and severe brown to dark brown shell staining, and mold contamination of shells and kernels (Michailides et al., 1995). ln some orchards, losses up to $1,000 have been reported by pistachio growers. Premature defoliation early (during August) may affect the crop of the next year and excessive defoliation during harvest may cause difficulties and problems for normal harvest processes. Other damages from Alternaria late blight include invasion of kernels by molds, resulting in low quality, moldy nuts, which lead to kernel rot and discoloration, off flavors, and perhaps mycotoxin contamination (Doster and Michailides, 1994b and Michailides et al., 1993).

In most of the countries in which irrigation is now practiced, diseases that were not present in non-irrigated crops have become common (Rotem and Palti, 1969). Alternaria blight has been severe in orchards where harvests are delayed and humid days and nights prevailed from late August through October. The disease shows up earlier in orchards irrigated by flooding although can become a serious problem in orchards irrigated with any type of irrigation system when harvests are delayed.

Susceptibility with age has been reported in nearly all Alternaria-host systems. In pistachio, we observed a similar situation: in individual trees, leaves on bearing shoots show Alternaria infections first while those on non-bearing shoots look green and healthy (with no lesions). An objective of our study tins year was to determine whether leaves on bearing shoots are more susceptible to Alternaria late blight than leaves on non-bearing shoots. In general, this difference may explain partially the different disease levels occurring during an on and an off year.

In previous studies, we found that spore inoculum populations of A. alternata increase from early August to mid September in pistachio orchards, regardless of their irrigation system (micro-sprinklers, sprinklers, or flooding), and defined this as the "critical period" for disease development (Michailides and Morgan, 1993). Using microloggers, we monitored the environmental conditions in the orchards and found that this is the time when relative humidity increases and dew formation begins in pistachio orchards.

In addition, the first symptoms of the disease appear during this period. Therefore, control measures should be done before this critical period to protect the susceptible leaves and fruit of pistachio. In research performed a few years ago, we reduced the levels of Alternaria disease by increasing the interval between irrigation so that the period from the end of July to 10 August was free of any irrigations (Michailides & Morgan, 199lb). For control of Alternaria late blight a combination of approaches should be attempted, and among the first should be manipulation of irrigation.

Pistachio orchards with soils of poor infiltration have been associated with reduced yields, primarily due to direct effects of tree water stress, and anoxia related to root problems are also common. In addition poor infiltration results in high relative humidity due to long-standing surface water and high levels of Alternaria blight. We hypothesized that by changing the flood irrigation to subsurface drip irrigation we could reduce the area of soil wetted by the irrigation water and thus affect the relative humidity, the duration of days with dew, and the temperature regimes in the orchard. Indeed, the 1994 results showed that our hypothesis was correct; for the first time, we showed that converting the flood irrigation system to subsurface drip was successifil in both reducing Alternaria late blight and increasing fruit quality by decreasing shell staining without affecting yields in a pistachio orchard in Kings County. In addition, subsurface drip increased the incidence of split nuts (Michailides et al. 1995). We report here the results of the second year‘s study.

Experimental Methods

Experimental plot This research was conducted in a 80-acre plot of 12–year–old Kerman (female) and Peters (male) pistachios in Kings County. The treatments were the existing flood irrigation and the new subsurface drip, buried at 28-30 inches deep. The ultimate goal was to create a subsurface wetting pattern that encompasses a large part of the root zone without wetting either the surface or the area extending below the root zone. Regular ½" nominal polyethylene tubing with 0.5 gallons per hour emitters every 18 inches incorporated in the line were used. The experiment included two treatments, the existing flood irrigation and the new subsurface drip in a randomized complete block design with five replications of 12 tree rows each of up to half a mile long. These extremely large replications are required to insure that humidity levels associated with the irrigation treatments will not be compromised by the air movement within the orchard.

Irrigation schedules were followed, using the water budget approach. Reference crop water use (ETo) and published crop coefficients (Kc‘s) were used to determine orchard water requirements (ETc). This information was adjusted to account for stored winter rainfall and for the limited surface evaporation in the buried drip treatment. While the goal was to irrigate the trees fully, the actual amount of applied water in the surface irrigation treatment depended upon soil infiltration and was at the discretion of the grower, who in turn, was advised by project personnel. The total applied water was 27.5 inches in the blocks irrigated by flooding and 26.9 in those irrigated by subsurface drip. The total amount of the winter rainfall was approximately 10 inches.

On 27 June we placed between the trees one each 21X Campbell micrologger in the middle of two adjacent blocks irrigated by flooding and two irrigated by subsurface. The environmental data (temperature, duration of 90 and 95% relative humidity, and duration of leaf wetness [dew]) recorded by the microloggers were retrieved monthly and processed with a computer to determine any differences in the environmental conditions created by the two irrigation systems in this orchard.

Incipient infections of Alternaria on pistachio leaves and fruit. Fruit and leaves were collected on 5 May and processed using the method described in last year's annual report (Michailides et al., 1995). Briefly, after surface sterilization, freezing, thawing, and incubation offruit and leaves for 7 to 9 days, colonies of Alternaria were determined. Freezing enhances the growth of incipient infections (Michailides et al., 1994), since all the fungal propagules on the surface are killed by the harsh sterilization approach.

Disease evaluation. The first symptoms of Alternaria blight on leaves appeared after 15 July, much earlier than in 1994. Thus, disease was evaluated in the field on 25 July, 24 August, and again 18 September on nine trees per replication in the center rows of each replicated block of trees. In addition, on 18 September 100 leaves were collected each from nine trees and evaluated in the laboratory for the number of lesions that had developed per leaf (severity of Alternaria late blight). To determine the disease severity index, infected leaves were separated in three categories as follows: 1, leaves with one to five lesions; 2, leaves with six to ten lesions, and 3, leaves with 11 or more lesions per leaf. Category 0 (zero) represented healthy leaves with no Alternaria lesions per leaf. Because symptoms on fruit appeared later than on leaves, the disease on fruit was evaluated only once at commercial harvest by collecting 30-35 fruit clusters each from nine trees per replication in each treatment. Sub–samples of 200 fruit from each tree were evaluated for infections by Alternaria blight. Fruit with disease was separated in 'infected,' showing portion of hulls infected and 'blighted,' fruit which had the entire surface of their hulls infected (dark brown or blackened)

Mycoflora of fruits and leaves. To determine the effects of the two irrigation systems on the spore inoculum of A. alternata and other fungal propagules, reaching and/or developing on the tree canopy, 10–leaf and 20–fruit samples were collected on 25 July and 25 August from each of nine random trees per replication. Leaves and fruit were washed and washings were plated in petri plates containing acidified potato–dextrose agar. Colonies of A. alternata, Aspergillus niger (which can cause Aspergillus blight), and other common epiphytic filamentous fungi and yeasts were counted.

Effect of fruit load on Alternaria blight. One hundred leaves were collected at harvest time each from shoots bearing fruit and shoots without fruit in each block and evaluated for Alternaria blight. The numbers of lesions per leaf were also determined.

Crop production parameters. Average yields were determined after harvesting all the trees in each block and all other crop production parameters from representative samples of harvested fruit. The crop production parameters are listed in tble below

The effects of subsurface drip irrigation and flood irrigations on crop production parameters of pistachio in a commerical orchard in Kings County

 
 
Gross dry yield (lbs/acre)
 
Split inshell (% dry wt.)
 
Total edible harvest
(% dry wt.)
 
split inshell
(lbs/acre)
 
Shell Stock
Light
(% dry wt.)
 
Stained
Dark
(%dry wt.)
 
Total insect damage
(% dry wt.)
 
Closed shell
(% dry wt.)
 
Removal of green nuts by weight mean
(% fresh wt.)
 

1994

Flood
 
1816
 
65.7
 
54.9
 
952
 
15.3
 
7.13
 
5.48
 
34.1
 
Drip
 
1545
 
82.4
 
71.2
 
1099
 
14.5
 
4.84
 
6.02
 
17.4
 
 
 
NSD 2
 
***
 
***
 
NSD
 
NSD
 
NSD
 
NSD
 
***
 

1995

Flood
 
3684
 
52.3
 
44.1
 
1614
 
13.3
 
4.52
 
0.96
 
47.7
 
Drip
 
3597
 
58.6
 
50.8
 
1813
 
14.5
 
4.92
 
0.96
 
47.7
 
 
 
NSD
 
NSD
 
NSD
 
NSD
 
NSD
 
NSD
 
NSD
 
NSD
 

Mean 1994 & 1995

Flood
 
2750
 
59.0
 
48.5
 
1283
 
14.3
 
5.83
 
3.22
 
40.9
 
Drip
 
2571
 
70.5
 
61.0
 
1456
 
14.5
 
4.88
 
3.43
 
29.4
 
 
 
NSD
 
***
 
***
 
***
 
NSD
 
NSD
 
NSD
 
***
 

1. Yields were based on harvesting all trees in each block and are the averages of five replications.
2. Statistical analysis using Duncan's New Multiple Range Test at P = 0.05; NSD = no statistically significant differences, and ***= statistically significant differences.

Statistical Analyses: Comparison of the effects of irrigation treatments was made using SAS statistical procedures. Because the experimental plot was arranged as a complete randomized block design, ANOVA was sufficient for determining significant effects of the treatments. Treatment means were compared with either the Least Significance Test (LSD) or Duncan's New Multiple Test at P =0.05.

Results and Discussion

Environmental conditions created by the two irrigation systems. Higher temperatures (by 1–1.5degrees C) were recorded in the blocks irrigated by drip than the flood-irrigated blocks. The mean temperatures in the flood and the subsurface-irrigated blocks ranged from 21.5-27.5 degrees C and the maximum temperatures from 31-37 degrees C in the drip-irrigated blocks and 30.5-36 degrees C in the flood-irrigated blocks (Figure 1). The mean daily relative humidity was higher (Figure 2), and the duration periods with 90 and 95% relative humidity were longer in the blocks irrigated by flooding than by drip irrigation (Figure 3A-B). These differences were expected because we noticed that after each flood irrigation the water remained on the surface for almost a week. Flood irrigation created longer wetness duration on leaves (hours with dew formation); in certain days there were about 5 hours difference between the two systems of irrigation (Figure 4). In general, these differences were greater just after the completion of irrigation, and were larger in late July than in August and September, which can explain why the disease symptoms appeared earlier in 1995 than in 1994.

Because of the standing water, the mean relative humidity and the duration of hours per day with relative humidity above 90% or 95% were greater in the flood–irrigated blocks than in the drip–irrigated blocks (Figures 2 & 4). Such differences in relative humidity were also recorded in last year's report (Michailides et al., 1995) and in previous studies (Michailides & Morgan 1992b). Although the blocks of the subsurface drip irrigation were 12-tree-rows wide, there was still a "bleeding" effect from the tree blocks irrigated by flooding (Figures 3 & 4). It is expected, therefore, that these differences to be larger when entire orchards are irrigated with either system of irrigation. In general, the subsurface drip irrigation created an environment among the pistachio trees that was less favorable for Alternaria late blight infection and development. Depending on the site of the block (south or north part of the field), blocks irrigated with subsurface drip had an estimated average of less than 25% of the soil surface wet while blocks irrigated by flooding, following each irrigation, had the water standing on the entire surface between the tree rows for several days.

Incipient infection of pistachio by Alternaria species. Similarly to the 1994 results, there were no significant differences in the levels of incipient infections in either fruit or leaves from trees irrigated by either flooding or subsurface drip. The same levels of incipient infection suggest that green fruit were equally susceptible to the development of incipient infections. Sixty seven to 76% of the pistachio fruit and 33 to 55% of the leaves (1.6 to 1.7 lesions per leaf) had incipient infections. Similar levels of incipient infections in fruit and leaves in the two systems of irrigation also suggest that the potential of disease was the same but, because of the differences in the environmental condition the two systems of irrigated had created, disease development was much higher in trees irrigated by flooding than those irrigated by subsurface drip irrigation (Table 1 and Figures 5 & 6).

Alternaria disease on pistachio leaves and fruit. The first symptoms of Alternaria disease appeared on leaves in late July (3 to 7% incidence) in trees of blocks irrigated by subsurface drip or flooding (Figure 5). However, disease incidence on leaves increased with time and increased more in blocks irrigated by flooding than in those irrigated by subsurface drip (Table 1 and Figure 5). At commercial time 55% of the leaves from the flood-irrigated trees had Alternaria lesions in comparison with only 10% of those from trees irrigated by subsurface drip irrigation (Table 1). In addition, leaves from trees irrigated by flooding had more lesions than those on trees from the blocks irrigated by subsurface drip (Table 1). The majority of leaves in the blocks irrigated by subsurface drip had only 1-5 lesions per leaf while about 40% of the infected leaves in the blocks irrigated by flooding had more than 10 lesions per leaf (Table 1). The levels of disease recorded in 1995 were very similar to those recorded in 1994 (Table 1).

Subsurface drip irrigation reduced more than five times both disease incidence and severity.

Evaluation of fruit at commercial harvest showed that more (about 37%) fruit was infected in the flood–irrigated blocks than in trees irrigated by subsurface drip (about 21%) (Figure 6).

We also noticed that some fruit were infected by Aspergillus blight but its incidence was less than 0.5%. In 1994, the incidence of Aspergillus blight was higher (up to 2.0%) probably because of the lower maximum (360 C) recorded in 1995 than in 1994 (37.50 degrees C).

Mycoflora on fruit and leaves. A number of fungi common on plant surfaces were recorded from plating washings from fruit and leaves of pistachio collected on 25 July and 25 August. These included A. alternata, Cladosporium spp., Aureobasidium pu11ulans, Botrytis cinerea, A. niger A. flavus, Epicoccum purpuracens, and various yeasts (Table 3). Fusarium and Penicillium spp. Were not recorded in 1995. There were no significant differences in the levels of propagules recovered from samples collected from trees irrigated with subsurface drip and flood. However, as a total about 38% more A. alternata propagules were washed from fruit and leaves of trees irrigated by flooding than those from trees irrigated by subsurface drip. In general, the total number of filamentous fungi and yeasts recovered from washings of pistachio fruit and leaf samples from trees that had been irrigated by drip did not differ significantly from the numbers of mycoflora recorded in samples from trees irrigated by flooding. Therefore, the differences in disease levels between the two irrigation treatments can be attributed on both, the Alternaria inoculum present on trees and the more favorable conditions created by the flood irrigation.

Effect of fruit load on Alternaria blight. The disease levels on leaves collected from shoots bearing fruit clusters from trees irrigated by either irrigation system were significantly higher than those collected from non-bearing shoots (Table 3). In addition, leaves from shoots bearing fruit clusters had as an average significantly more lesions per leaf Horsfall and Dimond (1957) introduced a theory of high- and low-sugar diseases and claimed that tissues which are low in sugar become susceptible to necrotrophic pathogens such as Alternaria which are associated with "low-sugar" diseases. Late in the season the non-reducing sugars disappear from the foliage, because they are drained into the ripening fruit. At this stage, the demand for non-reducing sugars exceeds their production by the green tissue and tenders the plant susceptible. The results of this study support this theory with Alternaria in pistachio. Leaves of shoots bearing fruit clusters are weakened and apparently become more susceptible to Alternaria infections.

These differences in Alternaria levels on bearing vs. nonbearing shoots suggest that when environmental conditions are favorable for disease development, it is expected that Alternaria late blight will be worse in on than off pistachio production years.

Crop production parameters. The results are shown in Table 4. Although there were no significant differences in the yields between the two systems of irrigation, a major increased in the percent split inshell nuts occurred in trees irrigated by subsurface drip. Surprisingly, there was no statistical difference in the stained nuts of shell stock although we recorded major differences m disease levels at harvest time. Generally, this low staining incidence could be the effect of the early harvest intentionally done to avoid major problems because of disease. It takes a few days for the shell stain to appear. Doster and Michailides (1995) showed that only nuts with brown staining are associated with mold colonization of interior shell surface and kernel. In this year's report, Doster and Michailides (1996) also showed that in both normal and crinkle-shell nuts, as the amount of general staining on the shell increased, the incidence of kernel decay also increased and the kernel quality decreased.

Summary
1. Converting flood to subsurface drip irrigation resulted in higher air temperatures, lower relative humidity, and shorter duration of dew formation.
2. Subsurface drip irrigation significantly reduced by more than five times the incidence of Alternaria late blight on leaves and by half infections on fruit.
3. Subsurface drip irrigation did not affect the levels of Alternaria spore inoculum on pistachio fruit and leaves neither the levels of the other mycoflora in comparison with the flood irrigation.
4. For two years, subsurface drip irrigation significantly increased split of inshell pistachio but did not affect yields.
5. Fruit load predisposes pistachio leaves to Alternaria late blight disease.

Therefore, based on results obtained in 1994 and 1995 subsurface drip irrigation is an effective method for lowering Alternaria ]ate blight in pistachio without creating any adverse crop production effects.

Acknowledgments We again thank C. Nichols for allowing us to perform this study in his orchard and J. Starling for the coordination of the irrigation schedule. We appreciate the help of L. Boeckler, K. Tsuda, A. Sullivan, M. Ruepp, A. Ziliani, and N. Flory. We also thank the California Pistachio Commission for the financial support of this project.
References Cited
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Doster, M.A., and T.J. Michailides. 1996. The use of shell discoloration to deternine kernel quality. California Pistachio Industry Annual Report, Crop Year 1995–
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