1. The most obvious benefit of regular orchard scouting is to let the grower know if his disease control program is giving satisfactory results.
2. The fungicides commonly used to control scab and most other diseases do not control powdery mildew or zonate leafspot. These diseases occur occasionally and require special control measures.  Regular scouting can aid a grower in knowing if and when these measures are needed.
3. Pecan varieties vary greatly in their susceptibility to scab.  Historically, the general trend has been for varieties regarded as scab resistant to become susceptible with the passage of time.  Many growers spray varieties believed to be scab resistant much less than varieties known to be scab susceptible. Orchard scouting can inform a grower if the disease control program used on a variety believed to be scab resistant is adequate.  “Stuart” is the leading pecan variety in Georgia. It has been considered more or less scab resistant for many years.  There has, however, been a continuous increase in scab on Stuart.  Some growers have suffered losses that could have been avoided with closer observation.  
4. The importance of pathogen populations becoming resistant to fungicides has been clearly demonstrated with pecan scab.  Regular scouting should provide early clues to problems of this nature.    

Symptoms:

Pecan scab generally shows up as small black spots on the new leaf parts (Figure 1).  Later, similar spots may be present on the small nuts (Figure 2).  With heavy infection, the lesions may grow together resulting in large irregular black areas on leaflets or nuts.  Early lesions may grow together resulting in large irregular black areas on leaflets or nuts.  Early infections may become large, 1/4 inch or more across, and sunken in the center by late season.  Old lesions, particularly in late season, are often overgrown by other fungi, giving them a white moldy look. 

Disease Development:

The pecan scab fungus overwinters on shucks and shoots infected the previous season.  Infections can occur as soon as green tissue appears in the spring.  They continue to occur throughout the season on any growing

Pecan Scab Evaluation:

When evaluating pecans for scab severity, observations should be made on a single variety.  In most older groves, Schley or Desirable will be the best choice.  For leaf scab, estimate the percent of leaflets with scab. For nuts, estimate the percent of shuck surface covered.

The rating system illustrated for pecan scab can be modified for use on all nut and leaf diseases of pecan. 

Pecan Scab Disease Cycle

Pecan Scab Grading

Symptoms:

Downy spot symptoms (Figure 3) are usually first seen in late June or early July.  The first symptoms appear as frosty white or pale green circular spots, about 1/8 inch in diameter, on the undersides of the leaflets.  The upper side of the leaflets may not show symptoms at this time. In mid-summer the lesions turn yellow. Small faded spots

Control:

Downy spot overwinters on leaves infected the previous season. Most infections occur before pollination but do not show up until late June to early July.  The key to downy spot control is a good prepollination spray program.  The varieties Stuart and Moneymaker are very susceptible to downy spot and may be used to check for its occurrence.  Fungicide treatments for downy spot after symptoms become obvious in July do not have much value and will not prevent drop of infected leaflets. 

Symptoms:

Powdery mildew first appears as dusty white spots, 1/8 - 1/4 inch in diameter, on nuts or leaves.  The symptoms are usually most obvious on the nuts which may become completely covered with dusty white growth.  Leaflets attacked early in the season often show large faded areas. Tissue development in these areas may cease causing the leaflets to become wrinkled and misshapened with age (Figure 4).  Nut infections (Figure 5) during June and July can lead to size reduction.  Nuts covered with mildew retain the potential to fill properly even when reduced in size. 

Most varieties can be attacked by powdery mildew, but Pabst, Wichita, Desirable and Farley are especially susceptible.  If any of these varieties are present, they can be scouted for powdery mildew.

Symptoms:

Zonate leafspot begins as small, gray to light brown spots on the leaflets.  These spots expand rapidly taking on a distinct target or banded appearance (Figure 6).  Symptom development and leaflet drop occur rather rapidly.  Leaflet drop may begin within a week of symptom appearance.

Zonate leafspot occurs on mature leaves.  All pecan varieties are susceptible to zonate leafspot. However, Schley appears to be more susceptible than Stuart.  The disease is most common in Calhoun, Dougherty, and Lee counties.  It has been seen in other areas of the state. In years when it occurs, its incidence may be very erratic from grove to grove.  Zonate leafspot has been observed to be more severe in trees bordering woods or in low areas of groves.  These sites tend to remain wet longer than open, level groves.  If a grower has had trouble with zonate leafspot in the past, he should treat for it when long periods of continuously wet summer weather occur.  Continuously wet weather is assumed to mean 4 - 6 days, but this has not been proven.     

Symptoms:

Fungal leaf scorch usually develops in late summer.  The scorch (tissue browning and drying) begins at the leaflet margin and expands inward.  There is a black or very dark brown band separating the scorched areas from the green portions of the leaflets.  Scorched leaflets drop prematurely (Figure 7). 

The exact cause of this disorder is unknown.  Fungi are believed to be involved because they have been isolated from the scorched tissue and because fungicides have given some control of this disorder.  Once fungal leaf scorch becomes obvious, there is little that can be done to control it.  It is important to keep in mind that not all leaf scorch is fungal leaf scorch.  Mites, nutritional disorders and dry weather all cause various kinds of pecan leaf scorch.  If leaf scorch appears, it is important to determine the exact cause so that corrective measures, when available, can be taken. 

Symptoms:

Brown spot only attacks mature leaves.  It is not found before June or July.  Infections appear as reddish brown, more or less circular spots. These spots cross over the leaflet veins.  Some spots may show grayish concentric rings at the margin.  As the spots age, they become irregular in shape.

Disease Development:

Brown spot is more common in areas of high rainfall and low fertility. Under these conditions brown spot can cause severe leaf drop in unsprayed orchards.  Trees properly fertilized and sprayed with fungicides are seldom bothered by brown spot.  Stuart is reported to be more susceptible than many other varieties.  Brown spot is most common in young orchards or orchards with no crop set as growers tend not to spray these sites as intensely as bearing orchard.