There is some confusion between necrotic leaf blotch and Glomerella leaf spot; however, it's important to know the difference due to different control strategies, since one is a physiological disorder and the other caused by a fungus.
Necrotic leaf blotch vs. Gomerella leaf spot
Necrotic leaf blotch typically appears "overnight" and after a cool, rainy period is followed by hot summer weather. This disorder is characterized by irregularly shaped brown blotches on the leaves (see figure 1). This is not a disease and the fruit will not be affected. The use of ziram has been shown to alleviate the symptoms in susceptible cultivars and to lessen defoliation. In contrast, the lesions associated with GLS start as purple spots, then grow with concentric rings, and may eventually coalesce (see figure). Symptoms may appear on both old and younger leaves and leaves may yellow and fall off.
Gomerella leaf spot (GLS) is caused by a fungus similar to that causing bitter rot on fruit. The moisture and temperature conditions that favor bitter rot development also favor GLS infection. Leaf infections can develop soon after bloom during warm, wet years, and leaves can be infected as few as 2 to 4 hr at 75 - 86°F; however, infection takes 16 hr at cooler temperatures down to 61°F. Disease severity increases proportionally as temperatures increase from 57 to 79 to 84°F.
The fruit rot phase of the disease is bitter rot. Symptoms usually appear on the side of the apple directly exposed to the sun as small, circular brown lesions that change to sunken, dark brown lesions as they enlarge. During humid conditions, large numbers of creamy to salmon colored spores are produced. Fruiting bodies visible to the naked eye appear after the lesion is one inch in diameter and are arranged in a concentric circle pattern in the center of the lesion. Beneath the lesion, the rotted flesh is watery and, when a cross-section is made, the lesion appears in a V-shaped pattern that narrows towards the core (see figure).
Figure 1. Characteristic symptoms of necrotic leaf botch, Glomerella leaf spot, and bitter rot. K. Peter.
The fungi causing fruit rots can be quite stealth since spores will land on the fruit and cause symptoms after the fruit have been in storage. We have had excellent conditions for fruit rots this month with frequent rain and stretches of warm weather. Growers are encouraged to apply a product, such as Merivon or Pristine (both 0 day PHI), that has efficacy against all of these rots. The materials also help with disease control during storage. Luna Sensation (14 day PHI) is another good option for controlling fruit rots; however, be mindful of the PHI. If possible, include one of these chemicals during your last cover spray in addition to captan. All of these products are also effective against Glomerella leaf spot.
Minimizing postharvest fruit rots
As you're getting your orchards ready for harvest, some general management techniques to keep in mind to reduce postharvest fruit rots:
- Bruised or wounded fruit are susceptible to blue mold and gray mold. While harvesting, handle fruit carefully when picking and transferring fruit from bag to bin to avoid bruising or wounding.
- The more mature a fruit, the more susceptible it is to storage diseases. Harvest fruit at proper maturity.
- Inoculum sources for rot pathogens causing disease in storage (if already not hitching a ride on the fruit), come from plant and soil debris. Use clean bins and minimize the amount of soil and plant debris brought in on bins.
- Warm temperatures encourage pathogens to grow. Keep fruit cool after harvest, i.e., keep bins in shade.
- If delivering to a packinghouse, minimize time between harvest and delivery of fruit.
For commercial growers, when controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from Crop Data Management Systems. Specific chemical recommendations are in: the Penn State Tree Fruit Production Guide