“Trunk disease” is a catch-all term that includes several different fungal diseases of grapevines trunks worldwide. The term was coined in the late 1990s by Dr Luigi Chiarappa ¹, and can include foliar and vascular symptoms caused by Petri disease, Black foot, Eutypa, Botryosphaeria dieback, Phomopsis dieback and Esca. Most of the fungi (more than 50 species) causing trunk disease are related, belonging to the order Botryosphareales, but fungi that belongs to the families that forms mushrooms can be also recovered from the cankers. In most of the cases, the cankers interfere with the movement of water from the roots to the leaves, shots and clusters.

Petri disease and Esca are caused by the closely related species of fungi in the genus Phaeomoniella and Paheoacremonium. In both diseases, longitudinal black stripes are visible under the bark. Different from Petri disease which occurs in vines younger than three years, Esca is observed in more mature plants. The foliar symptom can include the well-known “Tiger Stripe” pattern (photo 1). This symptom is visible normally by the end of June, when high temperatures stress the vines. Reddish-brown patches on the leaves are observed in red cultivars, while yellow patches are more common on white grapes. Esca is also known as “black measles” because the dark spots that develop on berries (photo 2). Measles is particularly damaging to table grapes because berry appearance is paramount.

Black foot is caused by two species of the fungal genus Cylindrocarpon, and it most commonly observed on young vines (three-year-old vines or younger). Stunted vines and scorched leaves (photo 3) are a typical sign of black foot. Affected roots present black lesions and look like they are dead. The disease was reported by late 1990’s in California and has been frequently found in fields with poor planting practices, especially those that were J-rooted at planting. The disease if not prevented, can require costly replanting.

Botryosphaeria is caused by more than 20 species of fungi. However, fungi in of the species Lasiodiplodia and Neofusicocum are the most damaging. Spurs from infected vines won’t develop new shoots (Photo 4) . In cross-section, a pie shape necrotic lesion can be observed in infected trunks (Photo 5), however this symptom can be also observed in vines infected with Eutypa.

Eutypa is a common disease in California, and it is caused by the fungus Eutypa lata. In addition to the pie shaped internal lesion, proliferation of stunted shoots is common (Photo 6). This symptom is absent in Botryosphaeria disease, where no growth or no leaf symptoms are observed.

Phomopsis is normally associated with damage on green tissue, specially canes and leaves. However, when conditions favor the pathogen, damage caused by Phomopsis can result in dead of spurs, canes and buds. Severely affected canes develop cracks and have a bleached appearance during winter. In early spring reproductive structures of the pathogen are visible as black speckles.

Impact
All the described trunk diseases can be seen at any time of the vineyard life, but normally they start to appear between the third and the fifth year after planting. They can appear alone, or in combination, which can exacerbate the plant stress. In general, by year 10 to 12, 20 percent of the plants show symptoms when no preventive actions have been implemented. At this point, trunk diseases enter in an exponential phase, reaching 75 percent of infection by year 15, and 100 percent by year 20.

Significant losses are associated with trunk disease development. Reduction in number of clusters, decrease in quality and cosmetic damages are the most visible impacts in the field. However, cost of replanting, and/or retraining if grower elects to use vine surgery, especially in young vineyards, it also a major cost associated with trunk diseases. Siebert2 in 2000, estimated the annual loss caused by Botryosphaeria and Eutypa in the California wine grape industry was $260 million. Similar negative effects have been reported in other grape growing areas and trunk diseases.

Management
Trunk diseases are considered chronic diseases, and unfortunately there is no fungicide that can provide curative action. Preventive management or the removal of infected tissue from the vine (surgery) and destruction by mulching or soil incorporation of infected shoots and canes are the only viable alternatives.

More than 95 percent of trunk diseases infections are associated with pruning, or other cultural practices that leave pruning wounds exposed at a time when the wounds may be infected. Dispersal of the pathogens responsible for causing trunk disease occurs during rain events. Under California conditions pruning and rain overlap during the winter months (November-January). Dr. Gubler and his team found that delaying pruning closer to bud break significantly reduces disease3. The logic behind this is that in a typical California year, rain is gone by the end of February and the days are warmer, letting the plants recover sooner than during the colder days of December and January. In addition, some sap movement (bleeding) starts to be present in February, helping the plant to remove the infective spores from susceptible tissue.

However, and knowing that the labor and logistics doesn’t permit all growers to postpone pruning until the last part of the winter, the use of fungicides to protect the exposed tissues is important to reduce the rate of the infection. The best practice is to protect the plant any time there are pruning wounds. This is especially important if rain is expected following pruning. If pruning is done in November or December, it is advised at least two sprays with protectant fungicides be applied. If the pruning is postponed until January and warmer days are forecasted, one protectant spray after pruning is ideal.

Another strategy for pruning is to do a double pruning (pre-pruning + pruning). It consists of pre-pruning the vines between November and January. Then, by the end of February or March, the pruning process is completed. The objective of this method is to remove any potential infection occurred during the winter months. A complementary fungicide spray after the last pruning can increase the control of trunk diseases.

In order to improve the efficacy of protective fungicide, a closer identification of the disease is recommended as any particular fungicide cannot control all possible pathogens. A recent report done by Baumgartner and Brown4 on their research in 2017, demonstrates that Pristine, Topsin + Rally (in mixture), and Luna had better preventive control of Botryosphaeria and Phomopsis dieback. Eutypa was controlled more effectively with Pristine. The highest control of esca was obtained with Serifel, but it only reached 64 percent. Results in 2018 in the same study presented different results and were mainly associated with a different weather condition.

Different studies, including the one recently done by Dr. Baumgartner and collaborators5, demonstrates that preventive practices works better if they are established during the first years of the crop (Figure 2). Dr. Baumgartner estimated that when more effective practices are adopted early in the crop life, it is expected to prolong the vineyard rentability by at least 25 years.

Further information on trunk diseases can be found at:

1) http://ipm.ucanr.edu/PMG/selectnewpest.grapes.html
2) Bettiga LJ, ed. Grape Pest Management, Third Edition. University of California, Agriculture and Natural Resources; 2013. https://books.google.com/books?id=4A9ZAgAAQBAJ.
3) Wilcox WF, Gubler WD, Uyemoto JK, eds. Compendium of Grape Diseases, Disorders, and Pests. Second. St Paul, Minnesota, USA.: The American Phytopathological Society; 2017.
4) Disease P, Gramaje D. Grapevine Trunk Diseases: Symptoms and Fungi Involved. 2018;102(1):12-39. https://apsjournals.apsnet.org/doi/10.1094/PDIS-04-17-0512-FE

Cited literature:

1. Gramaje D, Úrbez-Torres JR, Sosnowski MR. Managing Grapevine Trunk Diseases With Respect to Etiology and Epidemiology: Current Strategies and Future Prospects. Plant Dis. 2018;102(1):12-39. doi:10.1094/PDIS-04-17-0512-FE
2. Siebert JB. Economic Impact of Eutypa on the California Wine Grape Industry. Davis; 2000.
3. Gubler WD, Rolshausen P e., Trouillase FP, et al. Grapevine trunk Diseases in Califronia. Pract Winer Vineyard. January 2005:1-9.
4. Baumgartner K, Brown AA. Protectants for Trunk-disease management in California table grapes. In: California Table Grape Seminar. Visalia: California Table Grape Commission; 2019:19-21.
5. Baumgartner K, Hillis V, Lubell M, Norton M, Kaplan J. Managing Grapevine Trunk Diseases in California ’ s Southern San Joaquin Valley. 2019;3:267-276. doi:10.5344/ajev.2019.18075

Growers face a multitude of obstacles when trying to produce large volumes of high-quality strawberry fruit for a market that runs for many months. Up-front, pre-plant ground preparation and transplant costs are significant financial commitments that are expensed before even a single strawberry plant is put in the ground. Untimely rains and insect infestations can result in loss of fruit quality and numbers. A series of soilborne pathogens can later cause plant collapse and loss of profit. Of course, the intractable labor shortage dilemma may even result in perfectly marketable fruit not reaching the consumer.

When it comes to diseases of the strawberry fruit, the number one concern is gray mold (also called Botrytis fruit rot), which justifiably attracts the rapt attention of field professionals and captures the interest of researchers. However, in coastal California another fungal issue can also take its toll on strawberry yields and quality and in many ways is overlooked and underestimated by the industry. Rhizopus fruit rot and Mucor fruit rot are collectively known as “leak” disease; this disease concern also deserves to be recognized and studied.

Symptoms, Signs, Diagnosis
In the field, leak symptoms and signs only develop on mature or near-mature fruit and are very distinctive. Infected red fruit first take on a darkened, water-soaked appearance; in short order the fruit will begin to wrinkle and collapse. Almost overnight the rapidly growing Rhizopus or Mucor pathogen will be visible as white fungal growth peppered and interspersed with tiny black spheres. Fungal growth will be extensive and can entirely envelop the fruit, turning it into a white and black lump (Photo 1). Rhizopus and Mucor produce pectolytic (endopolygalacturonase) and cellulase enzymes as they colonize the strawberry, disintegrating the fruit tissues and causing red juices (Photo 2) to ooze and flow onto the plastic that covers the bed. It is because of these messy red juice flows that the designation “leak” is used for this disease.

This ugly scene in the field, however, is only part of the problem that these fungi cause. During harvest infected but symptomless fruit, as well as healthy fruit exposed to spores of the two pathogens, will be packed into containers. The pathogens can continue to grow during postharvest handling, storage, and market display of fruit, causing postharvest fruit losses, shortening of shelf life of the product, and creating a mess in crates and clamshells (Photo 3). In fact, if fruit are not properly refrigerated, the fungus on a single infected fruit can rapidly spread throughout an entire container, resulting in what is known as “nesting” or clumping of oozing, rotted fruit. Harvested strawberry fruit are subject to a number of rotting molds; the leak fungi are generally identifiable due to the color and nature of the fungal growth (see table).

The Pathogens
Much is already known about the two fungi causing strawberry leak disease. Rhizopus and Mucor are both in the group of fungi called Zygomycetes. Both fungi are commonly found in agricultural environments and cause similar ripe fruit rots on crops such as apricot, cherry, peach, pear, and tomato. While closely related to each other and difficult to differentiate in the field, the two pathogens differ slightly. On California strawberry, Rhizopus tends to be the more commonly encountered pathogen. Rhizopus grows very rapidly and haphazardly in orientation, creating a web-like mess of mycelium (Photo 4). Rhizopus forms a brown orange, root-like structure (called rhizoids) that allows it to quickly spread between adjacent fruit (Photo 5). The black, spherical spore bearing structures produce huge numbers of dry spores that are readily spread by winds (Photo 6). Mucor grows more slowly with an upright, erect mycelial habit (Photo 7) and also produces spores on a black spherical structure (Photo 8). However, Mucor spores collect in a wet droplet surrounding the head; such spores are therefore less prone to dispersal by winds.

Both fungi survive and increase in the field by colonizing dead organic matter and debris; Rhizopus and Mucor also readily colonize discarded and over-ripe strawberry fruit left on the plant or thrown into the furrow. These fungi produce a resilient structure (zygospore) that resists drying and weathering and provides another means of survival. Both Rhizopus and Mucor can be readily isolated from soil, thereby demonstrating that these fungi can persist in fields even if strawberry is not present. Once strawberry fruit begin to develop and ripen, spores come in contact with the fruit and usually gain entry via wounds and injuries. Strawberry leak pathogens tend to be more active if temperatures are relatively warmer, generally 65 F or higher. This temperature factor may be one reason that leak disease is more damaging in coastal California in late summer through early fall. The pectolytic enzyme that causes fresh market fruit to melt into juices can also be a concern in some processed fruit products. The enzyme is heat-stable and withstands canning temperatures; for example, apricot halves and brined cherries that are contaminated with Rhizopus and then canned may end up being apricot or cherry mush because of the continued activity of the stable enzyme even though the original fungus is cooked and dead.

Management Options and Research Needs
Growers already know to refrigerate strawberries as soon as possible after field packing the fruit. Refrigeration is a primary management tool for reducing losses due to leak disease. While refrigeration generally limits the development of Rhizopus, it is notable that some species of Mucor can grow quite well at storage temperatures of 32 F (0 C). Therefore, one research need is the precise identification of Rhizopus and Mucor species present in strawberry fields. Different species will have different temperature optima regarding growth and ability to infect fruit in the field, and the different species may respond differently to postharvest conditions.

Sanitation, both in field and during harvest, is another means of limiting leak development. Reducing the amount of over-ripe and rotted fruit in the field may help limit the Rhizopus and Mucor inoculum present in the planting; the influence of field sanitation on inoculum is another area of needed research. Field sanitation may be an increasingly difficult goal to attain given labor shortages and costs. Sanitation during the harvest process mainly involves the training and education of harvesters. Harvesters who touch and handle leak fruit will easily transmit the fungus to healthy fruit that are packed. Therefore, pickers should be reminded not to touch leak fruit and to be sure not to pack fruit showing any hint of leak infection.

Fungicides can effectively limit Rhizopus development for some crops. However, such information is lacking for strawberry. Research is needed to determine the efficacy and feasibility of using fungicides for leak management in strawberry. Other interactions involving fungicides should also be investigated. There are indications that fungicides used to manage Botrytis could exacerbate growth by Rhizopus and Mucor.

Among the many challenging factors facing strawberry growers, leak disease is not the number one concern. However, during certain times of the year in coastal California, leak disease can affect a significant amount of the harvest. A better understanding of strawberry leak disease, achieved through collaborative research, may help this industry manage this problem and improve on an already excellent commodity.

Neofabraea leaf and twig lesions were first detected in California super-high-density oil olive orchards in 2016. Since then the disease was found in Glenn, San Joaquin, and Stanislaus Counties. Causal agents of this new disease of olive were identified as Neofabraea kienholzii and Phlyctema vagabunda (syn: Neofabraea vagabunda). Phlyctema vagabunda is known in Spain as the causal agent responsible for the olive leprosy or lepra fruit rot, causing fruit malformation as well as leaf lesion and twig canker. This disease is of increasing concern in Spain, Portugal and Italy. Dr. Trouillas at UC Davis has outlined the disease epidemiology, disease cycle, and determined best spray timings and materials that will help to control this disease.

Disease Symptoms
Neofabraea leaf and twig lesions are primarily associated with wounds, such as those sustained during mechanical harvest. Leaf lesions are circular to elongate, necrotic, approximately 0.5 to 1cm in diameter and normally do not number more than one lesion per leaf (Figure 1). Twig lesions are reddish-brown in color mainly affecting the bark tissues (Figure 2). The disease may occasionally cause fruit rot near the time of harvest. In severely infected orchards, defoliation and fruit loss may occurr.

Disease Biology
Two fungal pathogens have been identified using morphological and molecular techniques: Neofabraea kienholzii and Phlyctema vagabunda (syn: Neofabraea vagabunda). These pathogens have been associated with bull’s eye rot and canker of apples and pears in the Pacific Northwest.

In olive, the disease has been detected primarily from super-high-density oil olive orchards in Glenn, San Joaquin and Stanislaus counties. The cultivar ‘Arbosana’ is the most susceptible but the disease has also been isolated on occasion from ‘Arbequina’ olives in the Central Valley. It was not found in the Koroneiki cultivar. Previous reports of the disease in California olive have included fruit spots in ‘Cortina’, ‘Picholine’ and ‘Frantoio’ varieties in Sonoma county. To date, table olive varieties (Manzanillo and Sevillano) in the Central Valley have not tested positive for Neofabraea leaf and twig lesions.

Infection occurs at the site of plant injuries. In super-high-density oil olives, these wounds are typically associated with damage caused by mechanical harvesters but may also include abrasion sites where leaves or twigs rub against each other. Following mechanical harvest, rain events allow for fungal inoculum to be released in the air, leading to infection of the fresh wound sites. Leaf spot symptoms are most visible in March, with defoliation occurring in April and May. Infected leaves and fruits act as inoculum sources for infection the following year.

Disease Management
Field trials have been conducted for three consecutive years in the highly susceptible Arbosana cultivar to determine fungicide efficacy. Results showed that several products were effective in reducing infection by the pathogens and limiting disease incidence. Overall, best disease control was achieved by Topsin M, Vanguard, Inspire Super, Bravo and Ziram fungicides, which provided up to 75 percent reduction in disease incidence. Copper fungicides did not control the disease. Comparison of different fungicide application regimes showed that one to two applications after harvest significantly reduce disease incidence. Two independent wound susceptibility trials were conducted also to determine the duration (0, 1, 2, 3, 4 or 5 weeks) when wounds on leaves remain susceptible to infection, and thus determine the number and timing of fungicide applications required to control Neofabraea and Phlyctema diseases. Results showed that leaves inoculated immediately after wounding (harvest) and those inoculated one week after wounding were the most susceptible to infection. Overall, leaf wound susceptibility to infection declined substantially after four weeks following wounding. This suggests that wounds had healed after four weeks following a wounding event at harvest and that one fungicide application after harvest followed by a second application two to three weeks later should suffice to protect olive trees from infection.

Next Steps
Two fungicides were nominated to the IR-4 program in 2018: Ziram (Ziram 76WDG) and difenoconazole/cyprodinil (Inspire Super). These fungicides were approved for residue trials at the National Food Use Workshop in September for registration on olives. Strong support was provided based on the after-harvest and winter season usage with expected zero to limit-of-detection residues on the crop in the following harvest season. Ziram is a FRAC Code M3 whereas Inspire Super is a FRAC Code 3/9. Thus, integration of multi-site modes of action for both products was also established as an effective anti-resistance strategy. Ziram and Inspire Super were also submitted for section 18 emergency exemptions, which are expected to come into effect during the course of 2020. The availability of these two fungicides in olive will improve control of Neofabraea and Phlyctema leaf and shoot lesions and will allow for management of fungicide resistance by rotating modes of action.

Acknowledgements
We are thankful to the Olive Oil Commission of California for funding this research.