33. Sunflower Downy mildew Identification and Life Cycle

33. Sunflower Downy mildew   
Identification and Life Cycle

Downy mildew of sunflower is caused by the fungus-like organism Plasmopara halstedii. The disease is initiated by soilborne dormant structures called oospores or infected seed. Oospores germinate in the spring in wet soils and produce swimming structures called zoospores, which migrate to roots, encyst, and germinate. Plants become increasingly resistant to infection with age, so systemic infections occurs over a short period (two to three weeks) after germination. The pathogen infects plants through roots, but becomes systemic within the plant. The pathogen produces zoosporangia profusely on roots and aboveground plant parts; zoosporangia are readily disseminated by wind and splashing water to other plants, where infection can occur through apical buds or leaves. The pathogen produces oospores in diseased tissues that are returned to the soil during tillage operations and serve as primary inoculum for future sunflower crops. Oospores can survive in the soil for five to 10 years. P. halstedii can also attack many weeds.

Plant Response and Damage

Downy mildew infected sunflowers are stunted with yellow leaves. White cottony masses of pathogen mycelium are apparent on the lower surface of leaves opposite the yellowed areas. As plants continue to grow, leaves become wrinkled and distorted. Heads of infected plants do not nod as they mature, but remain erect and produce little if any seed. The disease is most damaging when sunflowers are grown on heavy, wet soils with a history of downy mildew.

Management Approaches

Biological Control

No biological control strategies have been developed for downy mildew.
Cultural Control

Plant high quality certified seed free from the downy mildew pathogen. Practice a five-year or longer crop rotation between sunflower crops with nonhosts such as corn and small grains. Eliminate weeds that can serve as alternate hosts of downy mildew. Avoid reuse of irrigation water from sunflower fields, which can introduce oospores and zoosporangia into other fields. Provide adequate but not excessive irrigation, especially early in the season. Varieties resistant to some strains of the pathogen are available commercially.

Chemical Control

Seed treatments and fungicides are most effective when used in combination with cultural control strategies.

44. Grape Powdery Mildew 

Figure 1. Powdery mildew colonies on upper leaf surface of infected grape leaf.

The powdery mildew fungus can infect all green tissues of the vine. Small, white or grayish-white patches of fungal growth appear on the upper or lower leaf surface (Figure 1). These patches usually enlarge until the entire upper leaf surface has a powdery, white to gray coating (Figure 2). The patches may remain limited throughout most of the season. Severely affected leaves may curl upward during hot, dry weather. Expanding leaves that are infected may become distorted and stunted.
On young shoots, infections are more likely to be limited, and they appear as dark-brown to black patches that remain as dark patches on the surface of dormant canes.
If blossom clusters are affected, the flowers may wither and drop without setting fruit. Infections on cluster stems often go unnoticed, but can be very damaging. Infected cluster stems may wither and dry up, resulting in berry drop (shelling). Affected berries may have patches of fungal growth on the surface similar to those on the leaves, or the entire berry may be covered with the white, powdery growth. Infected berries often are misshapen or have rusty spots on the surface. Severely affected fruit often split open (Figure 3). When berries of purple or red cultivars are infected as they begin to ripen, they fail to color properly and have a blotchy appearance at harvest. Berries are susceptible to infection from early bloom through three to four weeks after bloom.
Late in the season, many black specks may develop on the surface of infected areas. These are the sexual fruiting bodies (cleistothecia) of the fungus.

Control
Select an open planting site with direct sunlight. Plant rows in the direction of the prevailing wind in order to promote good air circulation and faster drying of foliage and fruit. Prune and train vines properly in such as way as to reduce shading and increase air circulation.

Varieties differ greatly in their susceptibility to powdery mildew. Cabernet Franc, Cabernet Sauvigon, Chancellor, Chardonnay, Chelois, Gewurztraminer, Merlot, Pinot blanc, Pinot noir, Riesling, Rosette, Rougeon, Sauvignon blanc, Seyval, Vidal 256 and Vignoles are all highly susceptible.
On susceptible varieties, control is based on the use of properly timed applications of effective fungicides. Early season (prebloom through bloom) control of primary infections caused by ascospores must be emphasized.
54. Bacterial  diseases  of  cucumber

Basic requirements Cucumbers require warm, dry conditions to develop optimal ly, preferring both warm days and warm nights and growing best at a temperatur e of 30°C (86°F). Cucumbers will yield best if grown in a fertile, well-draining soil, rich in organic matter and with a pH between 6.5 and 7.5. Cucumbers are very se nsitive to cold and should be planted in full sun and provided with ample soil mois ture due to their shallow root system. Cucumber varieties One of the biggest co nsiderations when choosing a cucumber variety is whether or not it requires polli nating. Many newer cucumber varieties are gynoecious which means that they pr oduce only, or mostly, female flowers. Some gynoecious varieties require pollinat ing with male flowers, in which case a proportion of the seeds in the packet will b e pollinator plants which produce the male flowers. Some gynoecious varieties ar e parthenocarpic which means that they do not need the male plants to produce f ruit. These types are recommended for growing in glasshouses as they do not re quire the presence of insect pollinators. Sowing seeds Direct seeding is the pref

erred method for sowing cucumbers as they do not transplant well. Seeds should be sown after the last frosts and when the soil has warmed to at least 15.6°C (60 °F). Sow seeds 1.3–2.5 cm (0.5–1.0 in) deep, thinning to a spacing of at least 30 cm (12 in) between plants after germination. Cucumbers can also be seeded on hills or mounds of soil to encourage warm soil and better drainage. In this instanc e, seeds should be sown on hills in groups of 4–6 seeds, allowing 1.2 m (4 ft) bet ween each group in all directions. After emergence, thin the seedlings to 1 or 2 pl ants per hill. Cucumber seeds should germinate in 4–13 days depending on the s oil temperature. Cucumber have 4 kind of disease for example : fungus , bacterial , virus , Phytoplasma and caused by insect .
Bacterial leaf spot (Xanthomonas campestris)

Symptoms

Initial symptoms of the disease are the appearance of small water-soaked lesions on the undersides of the leaves which lead to the development of yellow patches on the upper leaf surface; the lesions become round and angular and may be mistaken for angular leaf spot; the centers of the lesions become thin and translucent and lesions become surrounded with a wide yellow halo.

Cause

Bacterium

Comments

Bacteria is spread via infected seeds.

Management

Use disease-free seed; do not grow plants in field where cucurbits have been

grown in the previous 2 years; avoid overhead irrigation, water plants from the base instead to reduce the spread of bacteria.

Bacterial wilt (Erwinia tracheiphila)

Symptoms

Individual runners or whole plant begins to wilt and rapidly die; infected runners appear dark green in color but rapidly become necrotic as the disease progresses.

Cause

Bacterium

Comments

Can result in crop losses of of 75%; spread by striped or spotted cucumber beetles; disease can be confirmed by cutting the stem and slowly pulling the two ends apart - infected plants will ooze strings of bacterial exudate.

Management

Control cucumber beetle populations on plants; hand pick adult beetles and destroy; soil and foliar application of appropriate insecticides may help to control

populations.

Angular leaf spot (Pseudomonas syringae)

Symptoms

Small water-soaked lesions on leaves which expand between leaf veins and become angular in shape; in humid conditions, lesions exude a milky substance which dries to form a white crust on or beside lesions; as the disease progresses, lesions turn tan and may have yellow/green edges; the centers of the lesions dry and may drop out leaving a hole in the leaf.

Cause

Bacteria, Bacterium

Comments

Spread through infected seed, splashing rain, insects and movement of people

between plants; bacterium overwinters in crop debris and can survive for 2.5 years.

Management

Use disease-free seed; do not grow plants in field where cucurbits have been grown in the previous 2 years; protective copper spray may help reduce incidence of disease in warm, humid climates; plant resistant varieties.