2006 Season Review: Fruit Diseases

Janna Beckerman, Department of Botany & Plant Pathology, Purdue University

It was a good year to be a fruit pathologist in Indiana!

The moderate winter temperatures of 2005 resulted in an above normal carry over of apple powdery mildew and increase in mildew infection, especially on Jonathan and other mildew-prone varieties. The moderate winter became a really wet spring in parts of the state, resulting in what seemed to be one continuous scab infection period from late March through May. This resulted in a bad year for apple scab, with fruit infection very high in those orchards lacking a good, early spray program for scab. This higher than normal infection rate has resulted in greater grower concern about resistance issues. Our preliminary data suggests two things: 1) These concerns are well founded, but 2) Timing of sprays could have been better. In the northern part of the state, unusually dry weather prevented serious scab development, but powdery mildew was higher than normal.

Wet weather in the central and southern part of the state brought a much higher incidence of cedar-apple (leaves and fruit), cedar-hawthorn (leaves only), and cedar-quince (fruit only) rust. The extremely wet weather this spring made for orange blobs, instead of the usual cute, or at least interesting "kooshballs" we usually see on junipers. Squishing the telial spore horns that release the spores that infect the apple leaves and fruit just wasn't as much fun as in previous years - in fact, it was downright gross.

The situation with fire blight is quiet. I haven't heard any reports of worse-than-usual outbreaks of fire blight, and in looking at orchards, I don't see them, either. Now I know the fire blight bacterium doesn't read the books, but with all the wet weather, and severe hail of 2006, it seems that this should be a bigger problem than it appears. It certainly is a bigger problem in the landscape! Come spring, be extra careful, and do a thorough scouting of cankers, just to be on the safe side. And in the spring, don't forget that dormant application of copper, too.

Sooty blotch and flyspeck of apple intensified as the wet weather continued throughout the summer. At harvest, both diseases were severe in orchards that had not maintained an adequate spray program. The strobilurins, used in combination with captan gives excellent control of this disease complex. Management of the other rots (bitter rot, white rot and black rot) remains tricky. Reports continue to come in on bitter rot control, particularly with respect to the 77-day PHI when using EBDCs. Current work is underway to address EBDC usage and Reregistration Eligibility Decision (RED) document.

You would've thought that the early wet weather would cause an increase in reports of Phytophthora crown rot on both apples and stone fruit. We thought so, too - and we were wrong. The constant moisture never caused any of the drought stress needed for symptom development. When that drought stress does occur, I suspect we will see an unusually bad year for phytophthora root and crown rot. For this reason, it is important to continue your treatment in the spring, with either Ridomyl, or phosphorous acid based fungicide (Aliette, AgriFos).

We had peaches this year! We also had tremendous outbreaks of every peach pathogen, due to the unusually wet conditions. I saw a lot of leaf curl on peach, but even more misdiagnosed peach scab. Many growers confused the symptoms of scab with bacterial spot. Peach scab primarily occurs on the shoulders, and doesn't cause foliar symptoms. Peach bacterial spot occurs on the fruit, leaves, and new growth. If the spots didn't get your peaches, brown rot might have, especially where mid and late season fungicide sprays had not been maintained. Fungicide resistance is an emerging issue with this disease as well.

Plum pox virus (PPV) was found in Michigan and New York this summer. In Michigan, the good news is that the one and only positive tree has been destroyed, and no additional trees have been found. The bad news is that the source of the PPV in the positive tree at SWMREC is still unknown. PPV is a plant disease infecting stone fruits, including plums, peaches, nectarines and apricots. The strain of PPV found in North America, PPV strain D, is less virulent than other strains, does not infect cherry trees and is not seedborne. Because the strain is not seedborne, it is not necessary to regulate the movement of fruit to prevent the spread of the disease. Several aphid species can serve as carriers of the virus. Symptoms of PPV infection include distortion and discoloration of fruit, yield reduction, and shortened lifespan of an already short-lived tree. The virus was first detected in Canada back in 2000, and was found in Pennsylvania in 1999.