June 19, 1997

In this issue

The following article, by Dr. Norman Lalancette, Rutgers University, appeared in the June 10 issue of Plant & Pest Advisory.

Observations at several commercial peach orchards, as well as in research blocks, reveal the presence of two powdery mildew diseases: rusty spot and powdery mildew. Incidence of these diseases this season appears to be higher than last year, hence this review.

Although the weather has been cool, it has also been relatively dry, which is favorable for powdery mildew fungi. These pathogens do not require free moisture to initiate infection.

Currently, the more prevalent disease is rusty spot, which is suspected of being caused by the apple mildew fungus, Podosphaera leucotricha. Early symptoms on peach fruit are small, circular orange to tan lesions. Since this apple fungus does not grow well on peach, it rarely sporulates and so lesions do not appear "powdery". These lesions enlarge as the fruit expands, eventually leaving a rusty-colored patch of dead epidermal cells.

Powdery mildew of peach and nectarine is caused by the fungus Sphaerotheca pannosa, which also causes powdery mildew on roses. Although this fungus infects leaves and shoots as well as fruit, disease has only been observed so far on fruit. Fruit lesions consist of white, circular spots that may enlarge and coalesce. Unlike rusty spot, powdery mildew lesions sporulate profusely, giving the lesions a three-dimensional, powdery appearance. As the fruit ages, the epidermal tissue becomes necrotic and appears scabby at maturity.

Disease control is implemented by application of fungicides and by managing inoculum sources on alternative hosts. Both of these actions should be performed to effectively control these diseases. Furthermore, any cultural practices that promote air movement and lower humidity are beneficial in reducing mildew infection.

Orchard sprays should begin at petal fall and continue through the early cover sprays on a 10-14 day schedule. Although fruit are only susceptible to powdery mildew until pit-hardening the effect of fruit age on rusty spot susceptibility is unknown. However, fungicide timing studies indicate that spray applications need to be performed at least until the second cover spray.

If you have these diseases, especially rusty spot, chances are you have been using sulfur for control. In a less weather-favorable year, sulfur would be adequate. However, the relatively dry spring combined with the mild winter results in above average conditions for infection. Under these circumstances, Nova is the best material for control of both diseases.

For peach rusty spot, powdery mildew should be controlled in nearby apple orchards since these orchards provide the inoculum. The sterol inhibiting fungicides Bayleton, Procure, Rubigan, and Nova are excellent for controlling mildew in the apple orchard. Also, peaches should not be planted adjacent to mildew susceptible cultivars such as Rome Beauty, Jonathan, Cortland, or Idared.

Rose bushes are an alternative host to peach and nectarine powdery mildew. If you have not noticed, look along the edges of any woods or hedge rows that border your peach blocks.

Chances are, you will find 'wild' multiflora rose bushes that are currently flowering (small white flowers in bunches). Many of these bushes are infected with S. panosa and are therefore inoculum sources for your peaches or nectarines. Either destroy these bushes or spray them with any of the apple mildew fungicides listed above. [Editor=s Note: Roses do not appear on the labels of any of the sterol inhibitor fungicides labeled for apple. Hence their use on roses would be contrary to label and illegal. - DAK]

The relatively mild winter may have allowed a higher population of the powdery mildew pathogens to survive on their alternative apple and rose hosts. Both pathogens overwinter as mycelium in dormant buds. Thus, management of these inoculum sources is important for keeping stone fruit blocks healthy.

If scab lesions are difficult to find now, how much fungicide is needed? Conditions that produce sooty blotch, flyspeck, black rot, white rot, and bitter rot infection are not well defined in terms of hours of wetting at various temperatures, as is the case for apple scab. Additionally, it has not been possible for growers to estimate the amount of inoculum for sooty blotch, flyspeck, and bitter rot in a way that can be correlated to spray requirement.

Therefore, recommendations have typically been aimed at maintaining fungicide residue throughout the summer, through regular applications every 2-3 weeks. Rains remove some of this residue, so the 2-3 week schedule is modified so that if 22 - 4 inches of rain falls before the 2-3 week interval expires, spray is re-applied. See Table 14 on page 62 of the New England Apple Pest Management Guide, or the January 17 issue of this newsletter for the normal application interval, and the rain threshold amount for various fungicides. 

The unusually dry air that has prevailed in much of Connecticut following bloom has not favored disease infection. Unless there is wet foliage, or relative humidity around 90% or higher, these diseases do not get started. In the continued absence of rain or high humidity, fungicides are not needed. It is quite unlikely that low humidity would persist for more than 2-3 weeks after a fungicide spray, but you should try to take advantage of it if that happens.

In the past, the decision about whether a treatment was justified for second generation tentiform leafminers (spotted tentiform leafminer and apple blotch leafminer) has usually been based on the number of sap feeding mines found on mature shoot-leaves. Various thresholds values have been suggested, from 2 to 12 mines per leaf, depending on apple variety, drought, other leaf damage, and previous experience. Vydate was the preferred insecticide for second generation, but because Vydate kills beneficial mites as effectively as it kills leafminers, second generation leafminer treatments have not been a preferred strategy. With Provado now available, the picture has changed somewhat. Provado does kill the beneficial beetle Stethorus punctum, a mite predator, but it does not destroy beneficial mites, according to research reports. However, Provado is reported to be most effective when used quite early in the development of a leafminer population; earlier than was needed with Vydate.

In view of these and other considerations, I prefer to make the spray-or-don=t- spray decision now, based upon the number of first generation mines. I do not know just what the threshold should be but in my Tolland orchard I find mostly 10-15 mines per fruiting cluster now, which is far too many to ignore. Fruit clusters have 4-6 infested leaves. I count all mines in all infested leaves in a cluster. My guess is that any block with 4 or more mines per fruit cluster, average, is a candidate for a Provado spray soon after second generation moth flight begins. For McIntosh, I would use a threshold of 3 mines per cluster. Moth emergence can be gauged by noticing how many, if any, of the leaf mines have an empty pupal case protruding from mines on the lower surface of infested leaves. If there is an exit hole from the mine, but no protruding pupal case (about 3 mm long), then either the leafminer was parasitized, or the pupal case was rubbed off by wind-blown leaf abrasion. Wind does not easily remove the pupal cases, and can probably be ignored unless a tornado or hurricane has gone through. Emergence has not yet begun in Tolland, but probably has begun in the southern one-third of the state by now.

Timing of a Provado spray in my orchard will be about 7-10 days after I see an average of one empty pupal case per fruit cluster, and again 7-10 days after I see an average of 6-8 empty pupal cases per cluster.

WHITE APPLE LEAFHOPPER has become quite noticeable now where populations are high, because many nymphs have matured to the adult stage, which is capable of flight. Do not waste money treating the adults; they are not easily killed. Instead, wait until a new generation of nymphs (can not fly) appears in July. Nymphs of rose leafhopper are expected to appear in early July, followed by white apple leafhopper nymphs in August. At that time a population of one nymph per leaf (average) probably justifies a treatment. Provado is the only effective material that does not also kill beneficial mites. The others are Carzol, Asana, dimethoate, Lannate, Thiodan, and Sevin.

If you can now find more than 5 or 6 motile (with legs) mites on any fruit cluster leaf, you will need a treatment within a couple of weeks, unless you find almost equal numbers of motile Zetzellia mali mites or unless you can expect Stethorus punctum beetles to move in. At this time, the options include Carzol, Kelthane, oil, and (if you are still within 4 weeks of Petal Fall) Agri-Mek. A Kelthane or Carzol application at this time will not end your mite problem, because those materials provide only short-term reduction of pest and beneficial mite populations. Oil is not a risk-free option if captan residues are on the tree, or will be used after the oiling. 

Where it is too late for good Agri-Mek absorption in problem orchards I see three strategies: (1) Wait for Pyramite to be approved for use in Connecticut; (2) use Kelthane or Carzol on only those varieties or locations where mites are of greatest concern, and wait for Pyramite for the less urgent situations; (3) use Kelthane or Carzol in one or two sprays when needed in June and early July in order to delay use of Pyramite as long as possible.

What distinguishes Pyramite from Carzol and Kelthane is that, even though it kills beneficial mites, and also Stethorus punctum, it is capable of relatively long-term control. The manufacturer (BASF) indicates 45 days of control can be expected. To avoid development of resistance, a second application in the same year is to be discouraged. If additional miticide is needed later this season, a decision can be made on the basis of miticides available at that time, and on the degree of success you have had with building a beneficial mite population.

The June 6 newsletter contained an error in regard to the time to look for SJS crawlers. Monitoring should begin soon after the middle of June.

In Tolland, SJS crawlers have not yet begun to emerge from the protective shells of their mothers.

Two species of aphids that commonly build colonies at this time of year are green apple aphids and spirea aphid. They are indistinguishable in the tree. Imidan and Guthion do not control these aphids, but insecticides are usually not needed in mature orchards if one allows commonly-occurring beneficial insects an opportunity to do the job. In young orchards where trees have not filled their allotted space, special insecticide treatment may be needed, because high populations of these aphids can stop shoot elongation. Lorsban, Thiodan or Provado may be justifiable if beneficial species (larvae of Aphidoletes, hover flies, and ladybird beetle, among others) fail to prevent serious aphid damage.

"Mildewcide sprays are applied primarily to prevent secondary infections. None of the fungicides will totally clean up primary infections. Leaves that are white now will stay white all season. When sterol inhibitor (SI) fungicides are used at 10-day intervals, they can prevent invasion of newly unfolding leaves on the ends of terminals with primary infections, but as soon as the SI applications stop, the mildew will appear on young leaves". - David Rosenberger, Cornell, Scaffolds, June 10, 1997.

To minimize cost of mildew control where it is a summer problem, it may be practical to target special SI or sulfur sprays just on those trees that require special attention, separate from the regular cover sprays. Make sulfur applications at 7-day intervals, or SI fungicides at 10 day intervals.

Because sulfur has not been widely used in Connecticut apple orchards in recent years, information on potential phytotoxic situations is limited. It is commonly thought that some varieties can develop leaf injury if sulfur is applied when temperatures are hot (above 80^oF ?), or if oil is used shortly before or shortly after the sulfur.

Benlate and Topsin M were once highly effective against mildew, but where they have been repeatedly used over the years, resistance is now common. I expect a similar loss of activity could occur with the SI's if they are used at borderline low rates and frequencies, allowing resistant strains to increase. In this regard, it should be noted that where active apple scab lesions are present, marginal low rates or frequencies of SI alone or along with marginal rates or frequencies of captan, will encourage the demise of SI's as scab fungicides.

The likelihood that a summer mildewcide program will be needed is greatly reduced if one has included an SI at label rates in sprays applied to cover the period from Bloom to 2 weeks past Petal Fall, according to Dr. Rosenberger. To eliminate the source of mildew spores, one can prune out the visibly infected shoots near the end of the growing season. Dormant pruning is less effective because it is difficult at that time to distinguish infected from healthy terminals. If effective pruning is done in infected blocks, the only other source of spores is infected apple trees outside the orchard. Other species do not harbor the mildew that infects apple.

Any fireblight that has appeared as shoot or fruit infection can serve as a source for additional infection if left in the orchard. For this reason, and also to reduce spread further in the original limb, infected fruits and branches or shoots should be removed as soon as possible. When trees are growing in spring and summer, the bacteria causing the disease moves well beyond the area of visible symptoms - often 12 inches or more. To reduce the risk of not getting it all, it is usually recommended that the pruning cut be made at least 18 inches below visible symptoms in the bark.

Pruning should not be done during wet weather, as the fresh cuts may be infected by bacteria carried in the rain. Most fireblight advisors also recommend that pruning tools be disinfected after each cut, using a cloth or sponge soaked with Clorox or other household chlorine bleach diluted to one tenth full strength.

When removing main limbs, leave at least 8 inches of healthy wood extending from the limb's point of origin. Come back and remove that stub in the winter, when the bacteria does not spread on pruning tools. Using this method, there has been less spread of fireblight into the scaffold limbs and central leader, than where flush cuts were made during the growing season.

If you have fireblight in your orchard, keep some streptomycin on hand in case a hail storm develops. Hail cuts in the bark or fruit can very easily be infected by fireblight bacteria, especially if rain accompanies the hail. Streptomycin spray should be applied within 24 hours of a hail storm in infected orchards: 4 ounces Agri Mycin 17WP, plus 1 pint Regulaid per 100 gallons water.

Once you are satisfied that plum curculio immigration into the orchard is finished, no further insecticide may be required until apple maggot (AM) season. The beginning of AM season is not the same time in all orchards of Connecticut. It should be determined on each farm by use of sticky lures. Several types of lures are available from IPM supply companies. The most widely used traps have been the dark-colored spheres that are thought to look like a large apple to AM flies. When coated with Tanglefoot stickum, these traps attract and hold AM, indicating the start of AM season. Insects other than AM will also be caught on these traps, so it is essential that you know how to identify AM. The wing pattern is sufficient to make the identification. The fly is the size of a medium or small house-fly.

Place traps about head-high in one or more fruited apple trees at the perimeter of the orchard block, facing any unsprayed apple trees on neighbors property, if such are present. A trap placed in an unsprayed tree will usually catch AM before they are caught at your orchard perimeter. Be sure the trap is placed so it is clearly visible from a distance, without obstruction within a foot or more. Check the traps every couple of days, removing leaves and non-AM insects that get caught. Cluttered lures are not very attractive to AM. Imidan or Guthion, or another effective insecticide should be applied soon after the first AM fly is caught on an unbaited sticky sphere. If no AM are caught by July 15, I recommend that an insecticide program, at 2 week intervals, begin anyway, unless you are willing to accept an unpredictable amount of fruit injury from codling moth, lesser appleworm, and various leafroller species that are present in July and August. 

David A. Kollas

Extension Pomologist

Phone: 860-486-1944

e-mail: dkollas@canr1.cag.uconn.edu