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In view of the frequent rains in numerous regions over the last few weeks, understanding rainfastness, or the ability of a pesticide to withstand rainfall, is important to ensure proper efficacy. All pesticides require a certain amount of drying time between application and a rain event. Typically, residue loss by wash-off is greatest when rain occurs within 24 hours of spraying. After this point, the rainfastness of a product will depend on formulation, adjuvants and length of time since application.

Rainfastness of Insecticides

John Wise, Michigan State University has studied rainfastness of various common tree fruit insecticide groups and is summarized below. For the complete article, click here.

Wise also presented this information during the 2021 Ontario Fruit & Vegetable Convention Virtual Educational Series:

Note that some products listed by Wise may not be registered for use in Canada. Check with your local supplier or refer to the 2021 Publication 360A: Crop Protection Guide for Apples for a complete list of registered products.

According to Wise, the impact of rain on an insecticide’s performance can be influenced by the following:

1. Penetration into plant tissue is generally expected to enhance rainfastness.
   • Organophosphates have limited penetrative potential, and thus considered primarily surface materials.
   • Carbamates and pyrethroids penetrate the cuticle, providing some resistance to wash-off.
   • Spinosyns, diamides, avermectins and some insect growth regulators (IGR) readily penetrate the cuticle and move translaminar (top to bottom) in the leaf tissue.
   • Neonicotinoids are considered systemic or locally systemic, moving translaminar as well as through the vascular system to the growing tips of leaves (acropetal movement).
   • For products that are systemic or translaminar, portions of the active ingredient move into and within the plant tissue, but there is always a portion remaining on the surface or bound to the waxy cuticle that is susceptible to wash-off.

2. Environmental persistence and inherent toxicity to the target pest can compensate for wash-off and delay the need for immediate re-application.
   • Organophosphates are highly susceptible to wash-off, but are highly toxic to most target pests, which means re-application can be delayed.
   • Carbamates and IGRs are moderately susceptible to wash-off, and vary widely in toxicity to target pests.
   • Neonicotinoids are moderately susceptible to wash-off, with residues that have moved systemically into tissue being highly rainfast, and surface residues less so.
   • Spinosyns, diamides, avermectins and pyrethroids are moderate to highly rainfast.

3. Drying time can significantly influence rainfastness, especially when plant penetration is important. For instance, while 2 to 6 hours is sufficient drying time for many insecticides, neonicotinoids require up to 24 hours for optimal penetration prior to a rain event.

4. Spray adjuvants that aid in the retention, penetration or spread will enhance the performance of an insecticide.

The following tables can serve as a guide for general rainfastness to compliment a comprehensive pest management decision-making process.

Table 1. General characteristics for insecticide chemical classes

Insecticide Group	Rainfastness ≤ 0.5 inch (1.25 cm)		Rainfastness ≤ 1 inch (2.5 cm)		Rainfastness ≤ 2 inches (5 cm)
	FRUIT	LEAVES	FRUIT	LEAVES	FRUIT	LEAVES
Carbamates (1A) Lannate	M	M/H	M	M	L	L
Organophosphates (1B) Imidan, Malathion	L	M	L	M	L	L
Pyrethroids (3A) Decis, Mako, Matador, Perm-Up, Pounce, Silencer, Up-Cyde, Ambush	M/H	M/H	M	M	L	L
Neonicotinoids (4A) Actara, Admire, Alias, Assail, Calypso, Closer, Clutch, Cormoran, Sivanto	M,S	H,S	L,S	L,S	L,S	L,S
Spinosyns (5) Delegate, Entrust, Success, TwinGuard	H	H	H	M	M	L
Avermectins (6) Agri-Mek, Minecto Pro	M,S	H,S	L,S	M,S	L	L
IGRs (15 & 18) Rimon, Cormoran, Confirm, Intrepid	M	M/H	M	M	L	L
Diamides (28) Altacor, Exirel, Harvanta	H	H	H	M	M	L

Table 2. Insecticide persistence, plant penetration and rainfastness rating

Insecticide Group	Persistence	Penetration	Rainfast rating
Carbamates (1A) Lannate	Short	Cuticle	Moderate
Organophosphates (1B) Imidan, Malathion	Medium-long	Surface	Low
Pyrethroids (3A) Decis, Mako, Matador, Perm-Up, Pounce, Silencer, Up-Cyde, Ambush	Short	Cuticle	Moderate-high
Neonicotinoids (4A) Actara, Admire, Alias, Assail, Calypso, Closer, Clutch, Cormoran, Sivanto	Medium	Translaminar, acropetal	Moderate
Spinosyns (5) Delegate, Entrust, Success, TwinGuard	Short-medium	Translaminar	Moderate-high
Avermectins (6) Agri-Mek, Minecto Pro	Medium	Translaminar	Moderate
IGRs (15 & 18) Rimon, Cormoran, Confirm, Intrepid	Medium-long	Translaminar	Moderate
Diamides (28) Altacor, Exirel, Harvanta	Medium-long	Translaminar	Moderate-high

Based on simulated rainfall studies to combine rainfastness with residual performance after field-aging of various insecticides, including carbamates (Lannate), organophosphates (Imidan, Malathion), pyrethroids (Capture), neonicotinoids (Assail, Actara, Admire), IGRs (Rimon, Intrepid), spinosyns (Delegate) and diamides (Altacor), Wise recommends the following re-application decisions for apples:

• ½ inch (1.25 cm) rainfall: All products with 1-day old residues could withstand ½ inch of rain. However, if the residues have aged 7 days, immediate re-application would be needed for all products but Assail, Rimon, Delegate or Altacor.
• 1-inch (2.5 cm) rainfall: In general, most products would need re-application following a 1-inch rainfall with 7-day old residues, whereas Delegate and Altacor could withstand this amount of rain on apples and would not need to be immediately re-applied. Some products such as Imidan could withstand 1 inch of rain with 1-day old residues.
• 2-inch (5 cm) rainfall: For all products, 2 inches of rain will remove enough insecticide to make immediate re-application necessary.

Rainfastness of Fungicides

There is no comparable research on rainfastness of fungicides and few labels provide this kind of information. A general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residue and over 2 inches (5 cm) of rain will remove most of the residue. However, many newer formulations or with the addition of spreader-stickers, some products may be more resistant to wash-off. However, avoid putting on fungicides within several hours before a rainstorm as much can be lost to wash-off regardless of formulation. As well, there are exceptions to the general rule in regard to truly systemic fungicides such as Aliette and Phostrol.

The effectiveness of sticker-spreaders with fungicides is variable and product/crop specific. Penetrating agents don’t help strobilurins; in fact, some fungicide/crop combinations have been associated with minor phytotoxicity due to excessive uptake. Captan, which is intended to stay on the surface, is notorious for causing injury when mixed with oils or some penetrating surfactants that cause them to penetrate the waxy cuticle.  Consult labels for minimum drying times for individual products and recommendations for using surfactants. 

Annemiek Schilder, Michigan State University suggests the following to improve fungicide efficacy during wet weather:

• During rainy periods, systemic fungicides tend to perform better than protectant (or contact) fungicides since they are less prone to wash-off.
• Applying a higher labelled rate can extend the residual period.
• Apply protectant fungicides such as captan (Supra Captan, Maestro), mancozeb (Manzate, Dithane, Penncozeb) and folpet (Folpan) during sunny, dry conditions to allow for quick drying on the leaves. These types of fungicides are better absorbed and become rainfast over several days after application.
• Apply systemic fungicides such as sterol inhibitors (Cevya, Nova, Fullback, Inspire Super), SDHI (Excalia, Fontelis, Sercadis, Kenja, Aprovia Top, Luna Tranquility) and strobilurins (Flint, Sovran, Pristine, Merivon) under humid, cloudy conditions. The leaf cuticle will be swollen, allowing quicker absorption. In dry, hot conditions, the cuticle can become flattened and less permeable, so product can breakdown in sunlight, heat or microbial activity or be washed off by rain.

For the complete article, click here.  

Summer Disease

In the last Apple Pest Update on June 26, 2021, I mentioned now is the time to start considering summer disease management. While symptom development doesn’t really start to appear for fruit rots, fly speck and sooty blotch until later in the summer or into the fall, infection actually happens now.

We have had ideal bitter rot infection conditions over recent weeks with the hot, humid weather followed by a rain storm. With hand thinning continuing, be sure to toss thinned fruitlets to the row middle and mulch. These fruit have been known to cause infection of remaining fruit on the tree by splashing spores during rain events.

Infection timing for fly speck and sooty blotch should also be considered. In wet years, infection typically begins 2-3 weeks after petal fall. With many regions so dry following bloom, it’s possible activity could have been delayed. There is a fly speck forecasting model that predicts the start of infection beginning 175 leaf wetting hours after petal fall. For many areas, this threshold has been reached.

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For management of summer diseases, follow a 14-21-day application interval until harvest to ensure fruit remains protected. Table 3 summarizes activity of fungicides that could be used for summer disease control. Residual protection will vary depending on the product used. For example, Pristine can provide reasonable residual control of summer disease up to 3 weeks at the labelled rate in ideal conditions; however, captan should not be pushed further than 14-days between sprays particularly since it is not as strong on fly speck and sooty blotch as other control products. In all cases, reduce interval to 7 days during times of hot, wet weather.

Table 3. Activity of registered fungicides on fly speck/sooty blotch, black rot and bitter rot

Product	Group	PHI	Scab	Fly speck / sooty blotch	Black rot	Bitter rot
captan	M	19 days*	+++(+)	++(+)	+++(+)	+++
Folpan (NR)	M	1 day	++(+)	++(+)	++(+)	+++
Senator (NR)	1	1 day	+++	++++	+++	+
Aprovia Top	3+7	30 days	++++	+++	—	—
Inspire Super	3+9	14 days	++++	+++(+)	—	—
Pristine	7+11	5 days	+++(+)	+++(+)	+++(+)	+++(+)
Merivon	7+11	0 days	+++(+)	+++(+)	+++(+)	+++(+)
Flint	11	14 days	++++	++++	+++	++(+)
Allegro	29	28 days	++	+++	+	+++
Phostrol	33	1 day	—	++	—	—
Regalia Maxx	P5	0 days	+(+)	+(+)	++	+(+)

Be sure to keep in mind preharvest intervals for all early and late-harvest varieties. Some products that may be most effective for summer disease cannot be used for fruit that will be harvested in coming weeks. At the same time, residues will need to be maintained on late-harvested varieties into the fall, especially if the hot weather trend continues in September and October.

It is also important to consider maximum seasonal application restrictions. While the label restriction applies to the active ingredient, one common resistance management strategy is to consider this restriction across the chemical group. For instance, the Pristine label states a maximum of 4 applications per year. For resistance management, where possible, try to limit this to 4 applications of products belonging to Group 11, not just Pristine. However, in an earlier year like the one we’ve had or during times of high scab pressure, this may not always be feasible.

Important cultural management recommendations include:

• Prune and thin clustered fruit to facilitate drying and improve fungicide coverage.
• Remove potential sources of inoculum such as dead branches, fruit mummies (where possible), infected fruit on the orchard floor and alternate hosts including brambles (blackberry, raspberry) from surrounding hedgerows or woodlots.
• Toss pruning cuts and thinned fruit in the row middle and mulch.
• Reduce tree stress by irrigating and/or applying a sun protectant, especially ahead of a heat wave.

Fire Blight

With terminal bud set, the cell walls thicken, stomata on the shoot stem are no longer active and the base of the shoot starts to harden off and form bark. All of which results in a significant reduction in fire blight movement. This is a good time to start to prune out fire blight strikes if you haven’t been already. Choose days of dry weather and leave the pruning cuts in the row middles to dry up before mulching them down.

The unsettled weather we are experiencing may continue to bring hail, strong winds or heavy rains that could still trigger trauma blight from terminal set until harvest. These could be very isolated events, occurring in one area but not another nearby. Assess for signs of damage in all blocks following any storm.

Be sure to apply Streptomycin (PHI 50 days) or 0.5-0.8% Cueva as soon as possible after the trauma event, especially if foliage damage is extensive or fire blight is known to be active in the orchard (ie., cankers, etc). New research is now suggesting ideal timing is 4 to 12 hours after a storm. Anything applied later runs the risk that the bacteria will have already established and begun to grow, reducing the efficacy of the product.

Efficacy of copper may be increased with the addition of Double Nickel.