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anthr Red Aleppo 8 Aug 24

(originally published November/December 2017)

 

Anthracnose of pistachio: In July 2016, putative diseased samples were collected from two pistachio (Pistacia vera) orchards in northern California (Glenn County) with black and sunken lesions on leaves and rachises. Samples were of the Red Aleppo, Joley, and Kerman cultivars. Eventually, individual fruit were totally blighted.  . These fruit blight symptoms looked different from the Botryosphaeria (Bot) panicle and shoot blight and did not bear any characteristic pycnidia of Botryosphaeriaceae fungi. Instead some of the fruit lesions developed slimy, pink ooze by harvest time. Lesions on the leaves were black and angular and also some developed the same slimy, pink ooze on the surface

anthr on alod new clean 1 Colletotrichum sp. from Calif cultures

(Photo 1).  .

Examination of the ooze under a compound microscope revealed masses of elliptical, one-cell conidia, characteristic of the fungal pathogen Colletotrichum. This fungal genus infects a large number of plant species, causing a plant disease generally known as “anthracnose” and can be very devastating because under the conducive environmental conditions can cause epidemics on various annual and perennial crops.  .

Isolations from spore masses of pistachio fruit and the margins of leaf lesions from multiple diseased samples revealed 100 percent Colletotrichum species recovery from samples the cultivar (cv.) Red Aleppo and 18 percent from samples of the cv. Kerman in the first orchard. In the second orchard, 9 percent of the isolations from fruit lesions of cv. Red Aleppo and 100 percent of isolations from small spots in panicles were Colletotrichum species.  . Also, 40 percent of the fruit lesions produced B. dothidea, an indication that the conditions were conducive to both of these diseases in the summer of 2016.  . By early August lesions showed on fruit and leaves of cv. Joly, which was also planted in the first orchard, and isolations from fruit and leaf lesions of this cultivar also produced Colletotrichum species at in the majority of isolations.  .

In Australia and China, the anthracnose of pistachio has been reported to cause significant yield losses, ranging from 25 percent to up 50 percent in years with wet spring and summer (Yang et al., 2012; Hall et al., 2014). In California, it is considered a new discovery. Specifically, a 50 percent destruction of the Australian pistachios was reported following the very wet summer of 2010. The occurrence of this severe disease in a couple of orchards in Butte County reminds us of how the devastating Botryosphaeria panicle and shoot blight started in a pistachio orchard in northern California, and in about 12 years the disease became a devastating epidemic on pistachio throughout the state.

History

Interestingly, anthracnose of pistachio was first found in a commercial orchard in Glenn County, in mid-July 1998 and by early September 1998 in a Kerman orchard in Tulare County, following the wet spring of 1998. This year (1998) was one of the years when Botryosphaeria panicle and shoot blight and the anthracnose of almond developed to epidemic levels in California pistachios. In this occasion, infected fruit were found covered with pink, initially slimy, then becoming powdery sporulation. The species C. gloeosporioides was morphologically identified as the causal agent, but no further investigations were pursued at that time since the disease did not spread following 1998. In the meantime in 2001, the disease was found and described in the pistachio cv. Sirora in Australia and attributed to the fungus C. acutatum. In 2004 and again in 2010, anthracnose symptoms were observed on fruit, leaves, and rachises in an orchard in Tulare County.

In 2010 and following the excess of rain and flooding in pistachio growing areas of Australia, the pistachio industry reported a 70 percent loss of the yearly crop. This tremendous yield destruction was attributed mainly to anthracnose, but also to some degree to Botryosphaeria panicle and shoot blight at a time when the mature crop was close to harvest and warm rains were in excess.

In California though during 2001 to 2015, and when the practice of checking Botryosphaeria levels in orchards by performing the bud monitoring bioassay (BUDMON) has become a routine test, sporadically Colleotrichum spp. have been isolated recovered from the plated dormant buds, suggesting that the fungus was present in some pistachio orchards.

In the summer 2015, Colletotrichum species were isolated from trunk cankers of pistachios declining mainly due to Phytophthora infections in a commercial orchard with heavy (heavy clay soils!) soil.  . However, the 2016 symptoms on cv. Red Aleppo were very severe and destroyed almost 50 percent of the yield of this cultivar. Initially the cvs. Kerman and Joley in the same orchard had lighter disease levels, but about two to three weeks before harvest the cv. Joley pistachios developed severe anthracnose symptoms on the clusters, similar to the disease of cv. Red Aleppo. In contrast, the nearby Kerman trees were still lightly infected by harvest, suggesting that perhaps this cultivar is more tolerant to the anthracnose than the cvs. Red Aleppo and Joly.

Symptoms

Initial symptoms appear as dark black circular or angular sunken lesions on the fruit and angular lesions on the leaves of about 5 mm in diameter. These lesions on both fruit and leaves expand in size and produce characteristic slimy pink sporulation

Anthr Red Aleppo 6

(photo 2).  .

When there are rains in the spring, the pathogen can also kill young taxianthies of pistachio (photo 3) and sometimes the tips of young and tender shoots. However, most of the damage is mainly because the fungus causes blighting of mature clusters resulting in mold and stained nuts.

anthr Joley 8 Aug 28

(photo 3)

Causal Agents  

      In 2016, two isolates from Tulare County and five isolates from Glenn County were reidentified initially to belong in the C. acutatum complex and C. boninense complex groups, based on morphological characters. Using molecular techniques though, the two isolates from the Kerman orchard in Tulare County were identified as C. karstii, while the five isolates from the Red Aleppo pistachios in the Glenn County orchard were identified as C. fioriniae. Because pistachio growers apply different kinds of fungicides on pistachio to control Bot and Alternaria blights, it is most likely that the disease epidemic in the orchards in Glenn County was a result of resistance selection among the Colletotrichum populations, which is frequent for this fungal genus.

      In order to understand the anthracnose disease cycle of pistachio in California, we will need to initiate proactive studies on managing anthracnose and the status of fungicide resistance in Colletotrichum populations in pistachio.

Epidemiology and Management

      Because Colletotrichum spp. produce slimy masses of one cell spores into a mucilaginous matrix, it requires rain and/or sprinkler irrigation to spread. In general, Colletotrichum species are able to grow from 50º to 85º F with optimum temperature for growth 75º F. Therefore, conditions in California pistachio orchards can be conducive to the diseases if inoculum and sufficient rains were present. Interestingly, new foliage that develops as a second flash after the rainy period is devoid of infection, although fruit can be infected by latent infections, which are established infections without showing any symptoms initially, but symptoms will develop later in season as the fruit matures before harvest.

      A fungicide trial was established in the spring of 2017 where fungicides were applied at label recommended label rates. The fungicides used included Merivon, KPhite, Inspire Super, and Tilt applied four times at almost monthly intervals starting at shell hardening (21 May) to July 20. All the fungicides resulted in lower levels of blighted fruit than the blighted fruit collected from untreated trees (control).This is the first fungicide efficacy trial against anthracnose of pistachio. We hope to include more fungicides in future trials in order to determine whether fungicides registered against Botryosphaeria panicle and shoot blight show also efficacy against the anthracnose.

      To answer the question whether anthracnose of pistachio could be a threat to the California pistachio industry, we can only say, that cv. Kerman is not completely resistant to infection, although it is more resistant than the greatly susceptible Red Aleppo, and Joley cultivars. Fortunately, not very many acres of these cultivars are planted in California. However, when we have excess of rains in the spring growers will need to monitor their fields for unusual symptoms of anthracnose. It does not take very long for a pathogen such as the anthracnose pathogen to reach epidemic levels, as it was the case with the anthracnose of almonds in California several years ago.  .   


By: Themis J. Michailides, Paulo Lichtemberg, Robert Sanders, and Juan Moral