Disease name : Root rot
Pathogen : Rhizoctonia solani (Thanatephorus cucumeris )
Life cycle:
The pathogen is transported in infested soil or through movement of diseased plants or peanut pods R. solani produces many sclerotia in host tissue. Sclerotia, which are aggregates of thick-walled hyphae, can persist in the soil in the absence of a host and will germinate when stimulated by exudates from a susceptible host or by the addition of organic matter to the soil. If the soil contains adequate organic matter, the fungus can grow saprophytically. Since many crops and weeds are susceptible to R. solani and organic debris is periodically added to the soil. long-term survival of the fungus may occur in many soils. R. solani produces cellulolytic and pectinolytic enzymes and phytotoxins that kill host tissue. As the tissue dies and disintegrates, it releases organic nutrients that promote continued growth of the fungus in the host.
Source of primary infection: Sclerotia, infected plants and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Sclerotia, infected plants and infected seeds or pods.
Symptoms: Peanut roots infected by R. solani initially have small, sunken lesions that are light to dark brown. On young seedlings, the cortex is decayed, and necrosis frequently envelops the entire root system, extending up the hypocotyl and killing the plant. Lesions on older plants usually are more prevalent near the soil surface. Often the most damage occurs on secondary and tertiary roots, where 1-2 cm of the root tips is decayed. The infected plant may live but be stunted and overgrown by adjacent plants. Root rots form at any time. Extensive root rots cause plants to decline and yields are reduced. This fungus also causes seedling disease and pod, and peg rots.
Control:
- Avoid excessive fertilization and irrigation which promote growth and create an ideal environment for disease development.
- Removing the debris after harvest will reduce the amount of inoculum in the soil.
- Alternation with a nonsusceptible crop to reduces inoculum level or follows three years crop rotations.
- Treat seeds by recommendation fungicides before sowing.
Disease name : Wilt
Pathogen name: Fusarium oxysporum, F .solani,
Life cycle:
Fusarium spp. including host-specific form species, live saprophytically in soil and reproduce on plant debris. Conidia are abundant but short-lived. Chlamydospores are the persistent survival structures. Symptomless carriers actively maintain during nonhost seasons, Seedborne inoculum and hyphae sheltered in slowly disintegrating debris also carry the pathogen over from one season to another. Injury to seeds predisposes seedlings damping-off and insufficiently drained, seeds may decay at the beginning of germination. Injury to roots predisposes seedlings to slow and suddenly willing. The fungus cause pod rots too. Under continuous peanut cultivation, the population density in soil increases considerably. However till abundance in soil is rarely accompanied by an epidemic increase of disease, with the exception of pod rots.
Source of primary infection: Spores, infected plants and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Spores, infected plants and infected seeds or pods.
Symptoms: Failure of seedlings to emerge from the soil (pre-emergence damping-off) or sudden wilt and death of seedling shortly after emergence (post-emergence damping-off) are symptoms of seedling disease. A reddish brown to black decay of the roots, or the stem near the soil line are commonly observed on infected plants. The upper leaves of infected adult plants turn pale green, yellowish and then die from the leaf margins inward. Infected plants become yellow, wilt and die gradually. A tan to brown colored discoloration of the vascular system is apparent when affected leaf petioles, stems, or tap root are cut and examined. Well-watered plants may survive for a long period, but the disease itself accelerates maturity.
Control:
- Removing the debris after harvest will reduce the amount of inoculum in the soil.
- Using of three years crop rotations.
- Treat seeds by recommendation fungicides before sowing.
Disease name : Charcoal rot
Pathogen name: Marcophomina phaseolina
Life cycle:
Charcoal rot is both seed borne and soil borne. Mycelium in seeds and mycelium and sclerotia in plant debris in the soil are primary sources of inoculum). The sclerotia can remain viable in dry soil for many years but rapidly lose viability in very wet soils. M. phaseolina is commonly present in peanut seeds and pods and can readily be disseminated by their movement. High soil temperatures (around 35°C) and low soil osmotic potentials reduce plant vigor and favor growth of the fungus and initiation of charcoal rot.
Source of primary infection: Sclerotia, infected plants and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Sclerotia, infected plants and infected seeds or pods.
Symptoms: The occurrence of water-soaked lesions of the hypocotyl near the soil surface is a characteristic sign of this disease. After the hypocotyl is girdled, the seedling dies. In older plants also, attack is most common at or near the soil line, although all plant parts at all stages of growth are susceptible. Stem and root lesions appear water-soaked at first, but infected tissues later become a dull light brown. The infection extends down into the taproot and up into the stem and branches. Where lesions girdle the stem, the plant wilts and the fungus rapidly colonizes the branches, which turn brown and die. The dead tissues rot and turn black, as sclerotia of the fungus develop profusely. Roots, pegs, and pods also rot and become covered with sclerotia. Insome cases, the disease is at first largely restricted to the roots, which become rotted and blackened with shredding of the taproot. The foliage of such plants turns yellow and wilts and the typical symptoms of stem blight and charcoal rot may appear.
Control:
- The promotion of vigorous plant growth with adequate irrigation and fertilization.
- Removing the debris after harvest will reduce the amount of inoculum in the soil.
- The disease most often occurs in fields rotated with grain sorghum.
- Treat seeds by recommendation fungicides before sowing.
Disease name : Crown rot
Pathogen name: Aspergillus niger
Life cycle:
A. niger is present in most peanut soils and is a common contaminant of peanut seed. However, outbreaks of the disease are sporadic and appear to be related to the prior occurrence of one or more stresses. Extreme heat or fluctuations in soil moisture during the seedling stage, poor seed quality, seedling damage from pesticides or cultivation, and feeding by root and stem boring insects are stresses thought to aggravate the disease. The economic importance of Aspergillus crown rot is difficult to assess. Generally scattered plants are affected, although stand losses of 50% have been reported in isolated fields.
Source of primary infection: Spores or sclerotia and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Spores, infected seeds and pods.
Symptoms: While the fungus may cause seed rot and pre-emergence damping-off of seedlings, the most obvious symptom is the sudden wilting of young plants. The crown area of infected plants just below the soil line may be swollen and eventually becomes covered with a black, sooty mass of fungus growth. Most affected plants die within 30 days of planting. Later in the season, individual branches or entire plants may develop similar symptoms. Splitting the crown and tap root of affected plants reveals an internal discoloration of the vascular system that is dark grey in color.
Control:
- Regular the irrigation in seedling stage.
- Using of three years crop rotations.
- Treat seeds by recommendation fungicides before sowing.
Disease name : Stem rot
Pathogen name: Sclerotium rolfsii
Life cycle:
Warm moist conditions favor disease development. The beginning of disease usually coincides with peg and pod formation, but under particularly favorable conditions, the disease may be initiated earlier. The fungus survives as a saprophyte on plant debris, even debris from non-host crops. The oxalic acid produced by S. rulf'sii kills plant tissue well in advance of the mycelium. Sclerotia survive from year to year near the soil surface but cannot survive for more than one year when buried deeply. In lighter soils, where oxygen concentrations might he greater, the fungus can he active in the pod region causing severe pod rot.
Source of primary infection: Sclerotia, infected plants and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Sclerotia, infected plants and infected seeds or pods.
Symptoms: First symptoms of the disease are usually the yellowing and wilting of a branch or, if the main stem is attacked, of the whole plant. Leaves turn dark brown. Sheaths of white mycelium can be seen at or near the soil line around affected plant parts. Mycelial growth is rapid under favorable environmental conditions and quickly spreads to other branches and plants. Produced abundantly on affected plant parts and the soil surface are at first white but later turn dark brown. Sclerotia produced by the fungus in culture are similar to those produced naturally. Lesions produced on branches and pegs are initially light brown, becoming dark brown as disease develops.
Control:
- Regular the irrigation in fruiting stage.
- Using of three years crop rotations.
- Removing the debris after harvest will reduce the amount of inoculum in the soil.
- Treat seeds by recommendation fungicides before sowing.
Disease name : Early and late leaf spot
Pathogen name: Cercospora arachidicola and Cercosporidium personaium
Life cycle:
Conidia produced on crop residue in the soil are the main source of initial inoculum. However, ascospores, chlamydosporcs, and mycelial fragments are potential inoculum sources. Conidia of C. arachidicola germinate, forming one to several germ tubes, which enter open stomata and penetrate directly through epidermal cells Cercosporidium personatum produces intercellular botryose haustoria, but C. arachidicola does not produce haustoria. During long periods of leaf wetness and temperatures ranging from 25 to 31oC, lesions develop within 10-14 days. Conidia are dispersed by wind, splashing water, and insects. Peak dispersal periods fur conidia occur at dew dry-off in the morning and al the onset of rainfall. Although vertical dispersal of C. arachidicola conidia to 9 ft above the soil surface has been reported, long-distance dispersal of C. arachidicola and Cercosporidium personatum conidia has not been documented.
Source of primary infection: Spores and debris plants
Pathogen spread: All peanut growing areas.
Means of spread: Sclerotia, infected plants and infected seeds or pods.
Symptoms: Early and late leaf spots are first recognizable as small necrotic flecks that enlarge and become light to dark brown spots ranging from 1 to 10 mm in diameter. Early leaf spot usually has a light to dark brown center and a yellow halo. The halo is often less conspicuous in or absent from late leaf spot lesions, but the intensity of the halo is not reliable for distinguishing between early and late leaf spot. Sporulation of the early leaf spot fungus usually occurs on the adaxial leaflet surface, whereas sporulation of the late leaf spot fungus is common on the abaxial surface. Although symptoms are useful diagnostic features, positive identification of early and late leaf spot requires microscopic examination of the conidia. Conidia are sometimes absent from early leaf spot lesions, but rarely absent from late leaf spot lesions. Lesions caused by both fungi develop on petioles, stipules, stems, and pegs in the later stages of an epidemic. Although the peanut is susceptible to a number of foliar pathogens, neither early nor late leaf spot is likely to be confused with other foliar diseases. However, some phytotoxicity symptoms caused by certain pesticides are very similar to symptoms of early and late leaf spots.
Control:
1. Regular the irrigation especially in sprayer irrigation case.
2. Using of three years crop rotations.
3. Removing the debris after harvest will reduce the amount of inoculum.
4. Spray foliar by recommendation fungicides on a 14-day schedule, beginning at 30-40days after planting and continuing until 14-21 days before the anticipated date of harvest, before sowing.
Disease name : Pod rot complex
Pathogen name: Rhizoctonia solani, Sclerotium rolfsii, Fusarium spp., Macrophomina phaseolina and Aspergillus spp.
Life cycle:
Pod rot is a term used to describe the decay of pods in soil. There are several causes of pod decay and discoloration in peanuts and several pathogens are a leading cause of pod rot. Advanced stages of pod rots result in complete decay of the pod and kernels. Yield losses can be substantial where a large percentage of the pods are affected. The fungi that cause pod rot are normally found at some level in most peanut soils. Likewise, plants with a few rotted pods are found in most fields. Severe outbreaks of pod rot are less common, but can be devastating where they occur. Pod rot tends to be severe in sandy soils and in fields intensively cropped to peanuts. A complex of factors in addition to the two fungi are probably responsible for these severe outbreaks. These factors include excessive soil moisture, wide fluctuations in soil moisture, calcium deficiency, insect and nematode feeding, and irrigation with poor quality (salty) water.
Source of primary infection: Sclerotia, spores, infected plants and seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Sclerotia, spores, infected plants and infected seeds or pods.
Symptoms: No apparent symptoms of pod rot appear above ground except that plants with severe pod rot may flower profusely and appear abnormally dark green in color late in the season. Below ground, symptoms of pod rot caused by Rhizoctonia solani give a light brown areas develop on pods which later turn dark brown or black A few to nearly all of the pods on one plant may be affected. Fusarium moniliforme usually causes a pink discoloration on rotted pods. While the other fungi cause pods have a general breakdown.
Control:
1. Regular the irrigation in fruiting stage.
2. Using of three years crop rotations.
3. Application of gypsum as a band over the row at flowering stage.
4. Removing the debris after harvest will reduce the amount of inoculum in the soil.
5. Treat seeds by recommendation fungicides before sowing.
Disease name : Yellow mold and aflatoxins
Pathogen name: Aspergillus flaves and A. parasiticus
Life cycle:
Aspergillus flaves and A. parasituicus are saprophytic inhabitants of many soils and can survive in crop residues. Their ability to cause plant disease is related to their competition with other soil microorganisms, the availability of susceptible plant tissues and the occurrence of favorable environmental conditions for growth, Aflatoxin contamination of peanut seed is influenced by the genetic composition of individual fungal isolates, composition of the substrate, competitive organisms, and environmental factors. The A. flavus group of fungi is more competitive when the soil moisture levels are below field capacity and the atmospheric relative humidity ranges from 90 to 98%, their ability to invade peanut pods and seeds are influenced by the physical and chemical composition of these plant parts. Some peanut cultivars have pods and seed coals with compact cells containing lignin. These cells function as barriers to nematodes, insects, and various fungi. Resistance to yellow mold thus depends upon maintaining intact pods and seed coats. Resistance is lost when cracks develop because of fluctuations in seed moisture, the activity of insects (such as termites), and rough treatment with mechanical equipment used for digging, combining, and conveying. These fungi grow over a temperature range of 17-42°C; the optimum temperature for aflaloxin production is 25-35°C, Fungal growth is optimum when moisture levels range from 15 to 30%. Levels maintained when the atmospheric relative humidity is 87-98%. Fungal growth on pods and seeds can be rapid under such drying conditions. When the atmospheric relative humidity is near 70% the seed moisture content will equilibrate between 7 and 9%, a level unfavorable for the growth of the A. flavus group. The time peanut pods remain within the optimum environmental conditions also influences the growth and production oflatoxins. Airflow rates of 10-30 ft per minute are capable of drying peanut pods to a safe moisture level for storage, provided the relative humidity of the forced air is below 70%. Heat may be added to the airstream to lower the relative humidity. Storage facilities should also be aerated with dry air to prevent moisture buildup in isolated spots.
Source of primary infection: Spores or sclerotia and infected seeds or pods.
Pathogen spread: All peanut growing areas.
Means of spread: Spores, sclerotia, infected seeds and pods.
Symptoms: Yellow mold first appears as spots on the cotyledons of peanut seedlings following emergence. Spots become covered with masses of yellow-green spore heads of the A. f!avus group. Infection may spread to the emerging radical and hypocotyl, causing necrotic lesions. Toxins produced by the fungus are translocated throughout the seedling in the transpiration stream. Infected plants generally become stunted and chlorosis on the leaflets. Infected seedlings lack a secondary root system, a condition known as aflarot. A majority of the infected seedlings recover from yellow mold when favorable growing conditions resume and persist. Invaded plants that remain under stress for several weeks are less likely to recover. The use of high quality seed treated with protectants greatly minimizes the severity of this disease. Yellow mold of peanut pods and seeds may first appear on or in peanut pods in the soil before digging, especially when plants are stressed by drought. Yellow-green Aspergillus colonies develop on pods and seeds that are overmature or on which damage provides openings for the fungus. Following digging, during the curing process, additional pods and seeds may become invaded, especially the immature pods. Following pod invasion, these fungi grow over the seed surface and may invade the seed to produce a hyphal colony between the cotyledons. Detection of such colonies (concealed damage) requires splitting the seeds. Once the seeds have been invaded, they develop a yellow to brown discoloration and weigh less than sound mature seeds.
Control:
- Regular the irrigation in fruiting stage and avoid drought stress and insect activity late in the growing season. .
- Practice crop rotation and proper fertilization to reduce the incidence of soilborne pathogens.
- Harvest when a majority of the peanut pods have reached maturity.
- Dry peanut pods with moisture contents above 9% by using forced air.
- Treat seeds by recommendation fungicides before sowing to reduce soilborne pathogens.
:المصدر
Dr.Emad El-Din Yousef Mahmoud
Plant Diseases Institute-ARC
د.عـمـاد الديـن يـوسـف مـحـمـود
معهد أمراض النبات- مركز البحوث الزراعية
ساحة النقاش