By the time you will read this article, pistachio nuts have almost completed 80% of the final size and their ovary is still small getting ready to start increasing its size to completion by the end of July. June is considered still too early to start seen early-split (ES) nuts. These will start showing in the second part of July and will continue developing even close to harvest.
June is the month when management of Alternaria late blight disease typically begins. However, before getting into the management of the disease let us understand, how, when, and why this disease develops in a pistachio orchard. There is a span of 2 months window (June and July) for the critical sprays that can control Alternaria late blight disease of pistachio. Alternaria late blight, as the name indicates, is caused by Alternaria species, the most common being Alternaria alternata. This species is ubiquitous in nature and can grow as pathogen in live tissues (leaves and fruit) and as secondary in orchard debris. In pistachio, the disease causes necrotic lesions in the surface of leaves where the pathogen produces abundant windborne spores under high humidity conditions in the orchard. A diagnostic feature of the Alternaria leaf lesions is the blackening of fingers after rubbing the sporulating lesions. Fruit also can be infected by Alternaria, especially those of late, second, or third harvests. In addition, if a rain occurs in late season, more disease will develop. That was the case during the tropical storm Hilary that occurred in the first half of August, resulting in severe outbreak of Alternaria late blight in pistachio orchards, even in areas historically characterized as “disease-free” under normal weather conditions in California (i.e., Westside). The fruit infection (decay) was so severe (Figure 1) that a grower admitted or reported? a 25% loss of his crop, although he had harvested twice this orchard. In addition to Alternaria blight, in the Terra Bella area where some orchards used to have some Botryosphaeria panicle and shoot blight, the disease developed in very high levels after the storm.
A Brief Summary on Disease Development
Although less frequent, Alternaria spp., like Botryosphaeria spp. can cause latent infection in the spring when rains wet the fruit and leaves and spores germinate to establish infections without any expression of symptoms. As leaves mature and start senescing, latent infections will be activated first producing very large necrotic brown lesions that will be covered with black spores. Wind mainly and rain spreads the spores causing a plethora of smaller lesions. The development of large and multiple small lesions per leaf results in defoliation of leaflets or entire leaves. Rain is not necessary for Alternaria to develop. High relative humidity in the orchard is sufficient for causing latent infection and expression of symptoms. Although Alternaria late blight develops symptoms initially on the leaves, fruit can also decay from Alternaria infection of the hulls, but by the first harvest, not much disease will have developed.
Susceptibility to Disease
If you have Golden Hills expect to have earlier and more severe symptoms than symptoms on Kerman. In an experimental field at the Kearney Agric. Research and Extension Center 3.5 acres of pistachios are planted in a design of 2 rows each Kerman alternating with 2 rows of Golden Hills. After infection and symptom development severe defoliation occurred in the Golden Hills rather in Kerman trees (Figure 2). We do not know the basis for the higher susceptibility of Golden Hills to Alternaria late blight, perhaps leaves mature earlier as does the fruit in this variety and thus the disease shows up much earlier and defoliates the trees excessively.
Weather Conditions Favoring Disease
As mentioned above high relative humidity favors Alternaria late blight disease and that is why under normal dry conditions in the summer the disease develops in certain areas with micro-climatic conditions. For instance, in certain locations where mature orchards may have overlapping tree canopies, ventilation in the orchard is reduced and the relative humidity is high, regardless of the orchard irrigation (drip or micro-sprinkler irrigation). A good recommendation then for new plantings is to plant the rows of trees N to S orientation so that when trees develop their full canopy, the N to S prevailing wind will move easier and faster throughout the orchard than when the rows of trees are planted East to West. In mature orchards, a wise recommendation will be to do hedging of the trees to open a ventilation path between the tree rows of the orchard. A late rain that will provide excess water on mature leaves and nuts will be detrimental, causing severe Alternaria late blight. Severe Alternaria late blight on nuts will result in not harvestable nuts, and in fact, those harvested will have severe brown staining of the shell, thus significantly reducing the nut quality.
Fruit Load & Alternaria Late Blight
In previous research, we showed that Alternaria lesions develop first in leaves of shoots that bear fruit; in the contrary, Alternaria lesions developed much later in leaves of shoots not bearing any fruit. Several years ago, we quantified the Alternaria disease as relates to the fruit load of shoots in an orchard in Kings County and found out that by mid-August, 45% of the leaves in shoots with fruit had Alternaria lesions, while at the same time only 11% of the leaves had Alternaria lesions in shoot not bearing any fruit. Also, in “on” production years, shoots heavily loaded with fruit start resulting in some yellowing of leaves, due to the high nutrient demand of the sink (fruit load). Thus, those stressed leaves are more susceptible to be infected by A. alternata. Therefore, it is expected to have more severe Alternaria late blight disease in “on” production years than in “off” production years. We suggest keeping the pistachio’s nutrients in optimal levels throughout the growing season.
Management of Alternaria Late Blight of Pistachio
Efficacy trials over multiple years provided results that helped the registration of many fungicides for the control of Alternaria late blight. These fungicides belong in five FRAC groups, 3, 7, 9, 11, and 19. In addition, pre-mixture fungicides that contain combinations of these FRAC groups showed excellent efficacy against Alternaria blight (see fungicide efficacy tables at https://ipm.ucanr.edu/legacy_assets/pdf/pmg/fungicideefficacytiming.pdf). One of the fungicides (Viathon) is a mixture of tebuconazole (FRAC 3) and phosphonate (P07, 33).
The discovery of new effective fungicides that belong to the FRAC groups mentioned above, usually leads to excellent disease control (rating 5) in the first 2-3 years. However, because most of these compounds are site specific and the resistance risk is high, eventually in subsequent years they become less efficacious in controlling Alternaria late blight. Despite the application of fungicides to control this disease, the frequency of Alternaria late blight samples brought to our lab by pest control advisors has increased exponentially in the last several years. This situation made us hypothesize that perhaps the resistance to the major fungicide FRAC groups might have become widespread. To find out if this contention was true, an extensive survey was initiated to determine what is going on. Samples of leaves (i.e. with lesions of Alternaria) (Figure 1) were collected from each of 20 orchards surveyed in southern and central San Joaquin Valley and the northern Sacramento Valley, brought to the laboratory and processed using the molecular qPCR procedure. Briefly, in this procedure lesions from each sample are blended, the DNA of Alternaria alternata is extracted, and by using specific primers, we can quantify the specific mutation(s) that confer resistance to the major groups of fungicides. For instance, the samples initially were processed to determine the levels of resistance to FRAC 11 fungicides, frequently used in pistachio orchards to control Alternaria late blight. Indeed, high resistance levels to strobilurins were detected mainly in the eastern side of south and central San Joaquin Valley and northern Sacramento Valley, while no or low levels of resistance were detected in orchards of the west side of the southern and central San Joaquin Valley (Figure 3). Similarly, when tests were done to determine the resistance levels to SDHI fungicides in diseased leaves from the 20 surveyed orchards, again high levels of resistance were detected (Figure 4). Interestingly, and very similarly to the resistance levels observed for stobilurin fungicides, the resistance levels to SDHI fungicides were lower in orchards of the west side of southern and central San Joaquin Valley (Figure 5). An explanation for these low resistance levels could be that usually growers in the Westside do not spray for Alternaria, since Alternaria late blight disease does not develop there (dry and windy, and “disease free” region!)
Alternaria late blight is very difficult disease to control because the pathogen is very prone to resistance selection to fungicides. In fact, because a major portion of Alternaria alternata population is resistant to two major FRAC groups of fungicides, a grower, a pest control adviser, or a consultant needs to be careful with the fungicide program to follow in order to achieve the maximum degree of disease control. Alternations of FRAC groups and pre-mixed fungicides can be used, along with at least one triazole (FRAC 3) in the program. In our 2023 fungicide efficacy trial experiment, we showed that the triazoles Cevya (i.e. mefentrifluconazole; registered) and the premixed Milbelya (i.e. mefentrifluconazole+ fluxapyroxad, FRAC 3/7; not registered yet), Luna Experience (i.e. tebuconazole+fluopyram, FRAC 3/7; registered) along with Miravis Prime (i.e. pydiflumetofen+fludioxonil, FRAC 7/12; registered) resulted in the least severe disease in an experimental orchard at Kearney Ag Research & Extension Center (Figure 6). This orchard has suffered from severe Alternaria late blight disease yearly.
Timing of fungicide Treatments
A grower or pest control advisor has three options regarding the timing of fungicide sprays for the control of Alternaria late blight. The first option is to apply one spray as an insurance coverage of the orchard. This should be done in the first week of July (Remember Independence Day!). Certainly, the decision of using one spray is based on historical data about Alternaria late blight disease. The second option would be 2 to 3 summer sprays. Sprays start beginning of June and end close to the end of July: 1st spray in early-June, 2nd spray in late June/early- July (best timing); and if high humidity is built up in the orchard due to the location and type of irrigation, then one more spray should be applied in mid- to late July. If any of these timings coincide with the timing of a NOW spray, you can combine these sprays in one, and thus save costs of tree spraying. Please do not spray in August and afterwards; instead of reducing the disease, you may create conditions to increase the blight.
The third option would be the use of a Disease Severity Value (DSV) model. For this there is a need to monitor leaf wetness and temperature in the orchard because combinations of those conditions determine when it is time for infections to occur, and therefore to spray. However, because not every grower has a weather station in the orchards and because this is more complicated to explain, in order to avoid confusion at this point, I will leave this discussion for a future issue of Pacific Nut Producer.
In summary, Alternaria late blight has become a major disease of pistachio in the last two decades, and it is a very difficult disease to control because the pathogen causing this disease, Alternaria alternata, is easily prone to fungicide resistance selection. The resistance levels were quantified in 2023 and found to be high and widespread among the main pistachio growing regions. Therefore, major caution is required when one decides on the fungicide program to control this disease. In addition, using an integrated control approach (cultural practices +chemical applications) will be the best option in controlling Alternaria late blight of pistachio. — By Themis J. Michailides, Boris X. Camiletti*, and Victor Gabri
