The extended wetting periods and cool cloudy weather we are experiencing in the lower Hudson Valley as trees are approaching petal fall create unusually high risks for quince rust, Botrytis infection, and fruit marking from phytotoxicity. Special cautions are advised.
Quince rust: Quince rust infects only fruit, not leaves. Fruit are at most risk of infection during tight cluster, pink, and petal fall, with relatively lower risk during bloom when I suspect the open petals make it difficult for spores to access the hypanthium below the flower (which eventually develops into the fruit). Unlike cedar apple rust, quince rust can attack all apple cultivars.
Quince rust in the Hudson Valley is most serious in years where we get extended wetting periods (greater than 48 hr) with temperatures above 45 F. during the time that trees are in late bloom or petal fall. It appears that this year the lower Hudson Valley may have more than 90 hr of wetting over a 6-day period when trees are nearing petal fall. The extended wetting periods allow more time for quince rust spores to travel from cedar trees into apple orchards. Heavy rains such was we have had over the past few days can knock some of these spores out of the air, but intermittent wet/dry periods without sunlight provide the best conditions for generating a spore cloud that will encompass the entire Hudson Valley. Infections under these conditions will NOT be limited to locations adjacent to cedar trees. Under conditions such as we are experiencing this year, I have seen quince rust infecting >60% of fruit in commercial orchards that were not adequately protected. Furthermore, infections can occur on flower stems, causing fruit on affected stems to drop from the tree.
Quince rust is relatively easy to control via applications of DMI (Group 3) fungicides. These fungicides provide excellent post-infection activity against all rust diseases on apples, with some evidence that they may provide 7-days or more of post-infection activity under some conditions. In the lower Hudson Valley, many location have had more than 2 inches of rain over the past three days, so any protection against rust diseases that might have been provided by sprays applied last week has been removed by rainfall. Thus, application of a Group 3 fungicide is advised when it becomes feasible to recover orchards because some post-infection activity may be needed to control quince rust this year. However, remember that post-infection activity will be limited to the plant tissue that is actually contacted by the spray, so spraying in windy conditions that compromise spray coverage may yield less that complete control of quince rust via post-infection activity.
Botrytis: Botrytis in many crops attacks fruit by invading dying petals and then advancing into the fruit or sepals that connect to the fruit. In apples, it appears to move from dying petals into the sepals where it usually remains quiescent until fruit have been moved to storage. During storage, Botrytis still develops slowly, usually appearing as a fruit decay only in fruit that are held until at least April. However, occasionally Botrytis can cause a blossom-end rot that appears before harvest.
Botrytis decays in stored fruit can be suppressed with postharvest drenches, apparently via eradication of the infections in the sepals (although this has not been carefully studied). However, in the mid-1990’s, before the introduction of 1-MCP and our newer postharvest fungicides, Botrytis sometimes caused postharvest fruit decays in more than 10% of Empire fruit that had been moved to CA storage without any postharvest treatment. I am uncertain whether fogging of postharvest fungicides or bin-top drenching after harvest can suppress Botrytis as effectively as the old recycling postharvest drenches did in the past.
Fungicides that have activity against Botrytis, if applied at petal fall, should help to minimize quiescent infections on apple fruit. (There is no data on this, because conditions that favor a high incidence of quiescent infections occur only rarely and did not occur in the years I attempt to investigate effectiveness of petal fall sprays for controlling postharvest decays caused by Botrytis.) Two products that have excellent activity (in the absence of resistance, which may be present in some locations) are cyprodinil and pyrimethanil (Vangard and Scala). The former is included in Inspire Super, so applying Inspire Super to control quince rust may also add some protection against Botrytis infection.
Botrytis can also infect stone fruit flowers and probably the shucks on fruitlets that have passed petal fall. In some cases, fruit abortion due to Botrytis infections in stone fruits may be mis-interpreted as poor pollination. There may be benefits to treating stone fruits this week with a fungicide that has activity against Botrytis as well as brown rot.
Fruit marking from phytotoxicity: Fruit and leaves on apples are most susceptible to pesticide phytotoxicity during the period from full bloom to first cover when terminal shoots are growing rapidly and fruitlets are just beginning to appear. Risks of phytotoxicity during this period are further increased if cool cloudy weather (as we are having in the Hudson Valley) persists for extended periods of time when trees are at this most susceptible growth stage. Sunlight, wind drying, and hot weather cause leaves and fruitlets to develop a thicker cuticle layer that minimizes risks of damage from crop protectants. However, in the absence of sunlight, drying, and heat, tender tissues can absorb pesticides that under normal conditions would not cause any injury.
In general, risks of phytotoxicity are increased by (i) slow drying conditions when sprays are applied, (ii) by complex tank mixtures, (iii) by addition of surfactants that increase uptake of toxic molecules, and (iv) by foliar nutrients that further soften foliage (i.e., urea) or that contain special surfactants to enhance uptake.
Captan is commonly mentioned as a potential cause of phytotoxicity. However, I have never seen captan cause phytotoxicity when applied alone on dry days. Usually, it is other products in the tank mix that cause captan to be absorbed into plant tissue. Thus, complex tank mixes during this period of peak environmental and growth-stage risk just add to the risk of phytotoxicity. I would prefer to avoid captan under these conditions and depend instead on alternatives until plant tissue hardens up a bit.
Carbaryl is often used at petal fall for fruit thinning, but carbaryl can cause fruit marking under certain conditions (e.g., slow drying, cloudy cool weather). Red Delicious are especially susceptible. Warnings about this are included on the product labels. The most common fruit markings that I have seen from carbaryl are tiny, black, blister-like spots that appear on fruitlets when they are about 1 inch in diameter. These can generate much concern when they appear in late May or June, but they become less noticeable by the time fruit mature and develop color. Nevertheless, the warnings are on the label, and it might be best to avoid carbaryl applications, if possible, under cloudy, slow-drying conditions.
