MOFGA Pest Report 2005
23 July

(View List Of All 2005 Pest Reports)

Root rot of peas

Many pea plantings are performing less than optimally this year and it may be due to root rots. Several species of fungi cause root rots and it is difficult to tell them apart without sending a sample into the disease diagnostic lab. Species of Pythium, Fusarium, and Rhizoctonia are common culprits. Damage is most severe in wet seasons and (again) problems in the fields now are really the result of conditions a month or more ago. At that time the soil was very wet, plants were suffering and more susceptible to fungi than they would be if they were growing well.

The above ground symptoms of pea root rots are yellowing of lower leaves, and then the whole plant turning yellow and dying. At later stages it is difficult to tell a root rot problem from a wilt problem. Below ground the rots are somewhat different from one another, but often more than one species of fungi is involved and then of course the symptoms are mixed. Here are descriptions taken from an Extension Bulletin of different root rots:

1. Aphanomyces root rot can infect plants at any age. The central part of the taproot (vascular tissue) separates easily as a long, fiber-like string from the outer, softened, water-soaked portion (cortex) of the root when the plant is pulled from the soil. The fine branch or feeding rootlets are killed. The decay may extend up the stem to slightly above the soil line. The decayed surface is often slimy in wet soils turning gray, then yellowish or pink, and finally brownish black as secondary microorganisms invade the diseased tissue. The leaves shrivel progressively upward on the stem. Plants are usually stunted, and the leaves progressively turn yellow starting at the bottom of the shoot. Pods may be few with a reduced number of small seeds. In severe cases, the plants collapse and die before forming any pods (Figure 1). Late-infected plants appear almost normal aboveground and often produce normal peas. Aphanomyces root rot is the most common and destructive disease of peas in the Midwest.

2. Pythium commonly causes seed rot as well as pre- and post-emergence damping-off of pea. Root rot of older plants also occurs, and often results in root-pruning that significantly reduces root length. Damage is most common in wet soils and is characterized by soft rot. Roots infected with Pythium are typically light brown in color and soft and watery to the touch. Infected plants are frequently stunted and pale green to yellow in color. Although it primarily causes a seed rot, damping-off, and seedling root rot, Pythium ultimum can cause a watery, soft decay of older plants in wet soils at an optimum temperature of 64° to 75°F (17° to 23°C).

3. Fusarium root rot affects mainly the taproot with infection starting close to where the seed is attached. Reddish brown streaks form in the primary and secondary roots and later merge. The external portion of the stem shows brick red, dark reddish brown, or chocolate-colored lesions. The advancing lesion may be wedge-shaped with the point upward. The central part of the taproot is a deep red. Plant growth is stunted, the foliage turns grayish, then yellow, the lower leaves wither, and the plant eventually dies. The lower stem is often girdled, causing the plant to fall over. Pythium ultimum is often found in Fusarium-infected roots and vice versa.

4. Rhizoctonia root rot can attack plants at any stage of growth. Seeds may turn dark brown and decay. Water-soaked, then reddish brown to brown lesions form in the seedling epicotyl and hypocotyl. The growing point may die as it emerges from the soil. Seedlings damp-off or recover to produce a normal plant. On older plants, scurfy, reddish brown, sunken lesions form on the underground stem and roots. The stem may be girdled causing severe plant stunting and yellowing. The brown, thread-like filaments (mycelium) of the causal fungus may be seen with a hand lens on the surface of the lesion or canker.

All of these root rot fungi live in the soil and can live long without a pea host. So, crop rotation works, but it has to be a long rotation (>5 years). The severity of the problem is centered around the weather. Wet soils and poor plant vigor leads to bigger problems.

There really is very little resistance in different varieties that I know of. If someone knows a variety or two that really are resistant let me know please.

Here are some things that I think will or may help:

1) Plant early in fertile, well-prepared, well-drained soil

2) Follow a crop rotation scheme in which peas are grown in the same field only once in 5 years or more.

3) Maintain an adequate to high, balanced soil fertility level, based on a soil test. The addition of at least 3 tons per acre of rock phosphate, or calcitic limestone may reduce losses from root rot. The optimum soil pH is 6.5 to 7.0.

4) Avoid overcrowding, deep planting, overfertilizing, soil compaction, and mechanical damage to the roots and stems.

5) A recent SARE project indicated that a rotation with hairy vetch/winter rye reduced the severity of pea root rot.

6) Biological controls such as Soilgard or Rootshield are claimed to help. Please let me know if you try these.

Tomato Leafroll

Leafroll is a common physiological disorder of tomato. It starts with the edges of the older leaves rolling up and inward to the extent that they may even appear tubular. This appears to be a means by which the plant conserves water during periods of environmental stess (hot and dry). Some cultivars appear to have a genetic predisposition to do this. The entire plant may be effected eventually and it is often permanent. But, the overall growth and fruit production usually are not impaired. To prevent this disorder growers should ensure that plants receive adequate moisture, especially during drought periods. And, in greenhouse situations, attempt to keep the humidity low to drive transpiration which cools the plant.

Diamondback moth

Diamonback moth adult

I have seen a very bad infestation of diamondback moth on kale in Cumberland County. This is the smallest of the caterpillar pests of the Crucifers. It is very small and easily overlooked until the damage gets out of hand. The caterpillars feed on the leaves of kale, cabbage, rape, and other Crucifers including mustard weeds. They prefer the undersides of leaves and do not eat the veins. The damaged skin tears as the leaf grows, creating holes and tears in the leaf.

(I've reprinted good pictures of the adult moth, larvae and pupae from the Cornell web site.)

Diamonback moth larvae

The adult moth is small and flighty and behaves more like a leaf hopper than a cabbage moth the way it quickly flies around the plants. When at rest it folds its wings on its back and is wedge shaped, and has three silvery white, diamond-shaped marks on the top. The larvae are tiny (just 8-10 mm), pale green and taper at both ends. The larvae pupate in loose, open-mesh cocoons on the leaves.

Diamonback moth pupae in cocoon

The diamond back moth overwinters in warm climates and moves northward by favorable winds. Eggs are laid on weeds if no crops are planted yet and hatch in 4-6 days. The young larvae mine into the leaves first, then older larvae chew the leaf surface. The mature larva spins a cocoon on the plant surface and new adults emerge in about a week. The moths are looking for mates in just a day and the next generation begins. There may be up to six generations in a season. Warm, dry weather favors the pest.

Heavy rains and overhead irrigation may reduce the number of caterpillars and if applied at dusk reduces feeding activity. After harvest, Crucifer crops should be disked into the soil burying all the foliage. Older plots will serve as inoculum of diamondback moth for new plantings. Later plantings should be in the direction of the prevailing winds to make it harder for the moths to fly into new plantings, and the greater the distance the better. Both Bt and Entrust have worked well. Alternating materials, if it is necessary to spray twice, is a good idea to reduce resistance build up.

Sweet Corn

Fall Armyworm Arrives in Maine
Corn Earworm Numbers Increase in Most Locations

Situation: Continued hot, mostly dry weather has helped corn develop rapidly in the past week, with many fields now coming into silk, and a few early fields starting harvest. There are still plenty of late fields around, however, and we continue to scout fields in the whorl to pre-tassel stage in most parts of the state. Fall armyworm moths were caught in one location this week, not causing too much alarm, but ending the mystery as to when this final member of the "big three" pests would arrive.

European Corn Borer: Moth activity jumped higher in most locations this week, as the first generation of the Iowa strain of European corn borer seems to be peaking quite a bit later than the earlier, New York strain. These moths pose a significant threat to silking corn in fields that are not yet being sprayed for corn earworm. This week sprays were recommended in silking fields in Cape Elizabeth, Jefferson and Nobleboro where more than five borer moths were caught, but which were still under threshold for corn earworm.

European corn borer larvae feeding injury was variable this week, with most fields still under threshold. Sprays were recommended for fields in Biddeford, Cape Elizabeth, Dayton, Lewiston, Oxford and Readfield on pre-tassel to tassel stage corn.

Corn Earworm: Moths have now been captured in most locations and populations have increased in some areas, leading to spray recommendations in most silking fields. This pest need only be controlled in silking fields, with spray intervals dictated by the concentration of moths captured each week. A single spray was recommended in Monmouth and Jefferson where the first moths were found in newly silking fields. A six day spray interval was recommended on silking fields in Dresden, Farmington, Litchfield and North Berwick. A five day spray interval was recommended on silking fields in Lewiston, Nobleboro and Readfield and a four day spray schedule was recommended for fields in Cape Elizabeth, Garland, and Wayne.

Fall Armyworm: Two fall armyworm moths were caught in a pheromone trap in Biddeford this week, announcing the arrival of this serious corn pest from its southern overwintering sites. As the season progresses we expect to this pest to show up throughout the state and pose a threat to all stages of corn. Armyworm moths prefer to lay their eggs on young corn plants. When the eggs hatch the young larvae chew channels in the leaves between the veins. As they get older and larger the feeding holes become larger and ragged, often surrounded by masses of wet sawdust-like waste. When found, this injury is combined with any European corn borer injury to determine if protection is needed. Control thresholds are 30% infestation in whorl stage corn or 15% for corn at pre-tassel and beyond. No fresh armyworm feeding injury was found this week, but we can expect to see it soon. On silking corn, moths may lay eggs directly on flag leaves or husks, and the larvae may move into the ears without leaving visible feeding signs to be picked up when scouting. Therefore, if not spraying for earworm, and three or more moths were caught in a week, a spray would be recommended on all silking corn.

Potato Leafhopper

There has been some potato leafhopper activity reported in southern Maine. Scout your potatoes and beans. Let me know if you find them.

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)