Written by Achour Amiri, Associate Professor, Washington State University, March 10, 2025
Sanitation reduces disease pressure and helps optimize the efficacy of chemical control
The apple powdery mildew fungus primarily overwinters in dormant buds, whereas pathogens of rots may overwinter on mummies, cankers, dead twigs, and leaf litter on the orchard floor. It is imperative to maintain proper sanitation within an orchard to mitigate disease pressure, reduce infection rates, and enhance fruit quality. The implementation of a comprehensive orchard sanitation program can significantly decrease the likelihood of disease outbreaks and lessen the reliance on chemical interventions. Diseased leaves and fruits that remain on the trees or the orchard floor (Figure 1) serve as reservoirs for pathogens, facilitating their persistence and spread. Given that many orchards were left unharvested in 2024, the risk of infections may be elevated in 2025.
Figure 1. Decayed unharvested apples and pears on trees and on orchard floors and leaf litter with mold development in commercial orchards. Photo Credit: Achour Amiri, WSU.
To effectively interrupt the disease cycle, it is essential to remove fruit and excess leaf litter prior to temperatures exceeding 50°F. Flail mowing represents the most efficient method for clearing orchard floors and eliminating the maximum amount of leaf litter. Click here for more on orchard sanitation.
Certain fungi induce cankers and dieback during the preceding season and winter. Manchurian crabapple pollinizers may serve as a source for speck rot and Sphaeropsis infections. Timely pruning and removal of infected or dead branches are essential to mitigate further spread of these pathogens. It is imperative that pruning tools are disinfected between cuts using a solution such as 10% bleach or 70% alcohol to prevent the transmission of pathogens. Additionally, wood cuttings and debris should be removed from the orchard, buried, or incinerated where permitted. Some pathogens have the potential to colonize weeds that grow in the alleys. Consequently, maintaining the orchard floor free from excessive weeds and tall grass not only diminishes habitats conducive to disease but also enhances air circulation, thereby aiding in the prevention of fungal infections. Furthermore, overhead cooling during the summer can lead to excessive moisture accumulation on leaves and fruit, which may promote fungal proliferation. It is crucial to adequately plan spray applications around cooling events to minimize runoff and to schedule the cooling events to avoid prolonged periods of wetness.
Protect young leaves and shoots early in the season against powdery mildew
Once the infected buds break in the spring as temperatures rise above 50°F, inoculum of powdery mildew becomes available and may be disseminated by wind to infect the newly emerging leaves, which are particularly susceptible. Infections are likely to persist throughout the growing season, especially during mild springs and early summers. Initial applications of fungicidal sprays should be conducted at the green tip stage, with subsequent treatments administered every 10 to 14 days in organic and conventional orchards, respectively, until the tree ceases to produce young leaves. More frequent applications may be necessary for highly susceptible cultivars during years with high disease pressure. As temperatures rise above 86°F, the incidence of infections typically declines. Click here for further information on mildew management.
Management of Storage Rot Pathogens Throughout the Growing Season
The semi-arid climate of Central Washington inhibits the manifestation of disease symptoms within orchards; however, it is estimated that up to 70% of storage rots originate in orchards. Throughout the growing season, fruits may be infected by various pathogens, which can subsequently transmit infections to storage facilities, where symptoms may not become apparent until weeks or months later. Implementing Integrated Pest Management (IPM) strategies, particularly through sanitation measures at early growth stages and consistently throughout the season, can significantly mitigate disease pressure and reduce the incidence of infected fruits at harvest. Nonetheless, the application of fungicides remains essential for further decreasing infection rates and, consequently, the occurrence of storage rots.
Petal Fall and Preharvest Sprays: Essential Strategies for Mitigating Storage Rot
The petals and senescent floral parts are susceptible to infections from various pathogens, including Alternaria spp., Botrytis spp., and Fusarium spp. Some of these infections may persist and transfer to the developing fruits, resulting in dormant infections that can later manifest as core rot within the seed cavities during storage. Consequently, it is imperative to mitigate early-season infections through the application of fungicides, including fungicides from the FRAC groups 1, 7, 7+11, or 17 (labeled for pear only), as FRAC group 3 fungicides are more effective against powdery mildew rather than storage pathogens.
As fruits approach harvest, their susceptibility to infections increases, a situation that can be exacerbated by elevated inoculum loads later in the season, favorable climatic conditions such as milder temperatures beginning in mid-September, and rainfall affecting late-maturing cultivars. Additionally, unnoticed microcracking on the fruit surface can further heighten the risk of infection. Therefore, it is crucial to implement protective measures for the fruit during this critical period to prevent additional infections.
Harvesting at the optimal stage of commercial maturity while minimizing wounds and punctures during the harvesting process is vital to reducing the risk of infections that typically lead to rapid decay, particularly from Penicillium spp. and Mucor spp. To prevent the selection of fungicide-resistant populations, it is highly advisable to rotate fungicides from among FRAC groups 1, 7+11, 17, and 19 (labeled for pears only). These fungicides are anticipated to provide residual efficacy throughout the extended storage period.
Fruitlet Stage and Summer Spraying to Mitigate Storage Rot
During the fruitlet stage, the onset of new fruit set may be accompanied by the reactivation of pathogens carried over from flowers and petals, potentially leading to dormant infections within the developing fruit. Implementing a fungicide spray at this interval could significantly mitigate the risks associated with these dormant infections, particularly those related to the calyx and stem ends. In mild-springs and summer conditions, pathogen populations can proliferate, resulting in increased inoculum levels and subsequent infections. A targeted summer spray would effectively interrupt the infection cycle and inhibit excessive inoculum accumulation as the fruit progresses toward ripening. It is advisable to alternate fungicides from the FRAC groups 1, 7, 7+11, or 17 to enhance efficacy and manage resistance.
Disease Management in Organic Orchard Systems
Pathogens exhibit comparable epidemic patterns in organic orchards relative to conventional orchards. A significant distinction lies in the fact that disease pressure is often markedly higher in organic orchards, which elevates the risk of infections and results in a greater likelihood of fruit contamination at the time of harvest. Given that organic packers currently lack access to effective postharvest fungicides, as their conventional counterparts do, it becomes imperative to optimize disease control preharvest. Sanitation measures assume critical importance in organic orchards to eliminate sources of inoculum and mitigate disease pressure. Additionally, it is essential to harvest organic fruit at commercial maturity; a delay of even one week can substantially increase decay rates during storage.
It is recommended that growers implement at least three to four spray applications per season at the designated stages, in conjunction with sanitation practices. A variety of organic materials, including biological agents, plant extracts, salt-based products, and Polyoxin-D (OSO SC 5%), are available for rotation throughout the growing season (Table 1). Because these organic materials do not possess similar efficacies against various pathogens, it is vital to alternate between multiple products or employ tank-mixing strategies, when feasible, to enhance overall effectiveness. Research indicates that the application of organic sprays within the preharvest interval (7-0 days prior to harvest) is crucial, and it is advisable to employ the most effective materials during this critical phase.
| Mode of Action | Trade name | Active ingredient (a.i.) | PHI1 | Labeled for Mildew | Labeled for Rots | Efficacy in WA2 |
|---|---|---|---|---|---|---|
| Antibiotic | OSO 5%SC | Polyoxin-D Zinc Salt | 0 | + | + | ++++ |
| Sulfur | Lime Sulfur | Ca polysulfide | 0 | + | – | N/T |
| Biological | Howler | Pseudomonas chlororaphis strain AFS009 | 0 | + | + | N/T |
| Biological | Botector | Aureobasidium pullulans strains DSM 1494/14941 | 0 | – | + | ++ |
| Biological | Actinovate AG | Streptomyces lydicus WYEC 108 | 0 | + | + | ++++ |
| Biological | AVIV | Bacillus subtilis strain IAB/BS03* | 0 | + | + | ++++ |
| Biological | DoubleNickel | Bacillus amyloliquefaciens strain D747 | 0 | + | + | ++ |
| Plant extracts | Cinnerate | Cinnamaon oil/Potassium oleate | 0 | + | + | +++ |
| Plant extracts | Guarda | Thyme oil | 0 | + | + | N/T |
| Plant extracts | Regalia | Reynutria sachalinensis | 0 | + | + | ++ |
| Plant extracts | Problad Verde | Banda de Lupinus albus doce (BLAD)* | 1 | + | + | +++ |
| Copper | Cueva | Copper Octanoate | 0 | – | + | ++++ |
| Copper | Instill-O (apples only) | Copper Sulfate Pentahydrate | 0 | – | + | N/T |
| Salt | Carb-O-Nator | Potassium bicarbonate | 0 | + | + | ++++ |
| Oxidizer | Jet-Ag | Hydrogen Peroxide/Peroxyacetic Acid | 0 | ? | + | ++ |
1 Preharvest interval
2 Overall efficacy observed over three seasons on Fuji apples in Rock Island. Efficacy may differ based on cultivar, growing region, or environmental conditions. Always read the label before applying.
Contact
Achour Amiri
WSU Tree Fruit Research and Extension Center
a.amiri@wsu.edu
(509) 393-4058
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YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.
