Disease models - chilli, eggplant and paprika - METOS® by Pessl Instruments

Chilli, eggplant and paprika disease models

Pepper is an important commercial vegetable crop worldwide. Production of pepper is often severely limited by one or more disease problems. This content describes the symptoms of several commonly observed diseases and the models used for its prediction in FieldClimate.

ALTERNARIA AND TOMCAST -
Alternaria solani

POWDERY MILDEW
RISK MODEL

GREY MOULD -
Botrytis cinerea

PHYTOPHTHORA BLIGHT -
Phytophthora capsici

Alternaria and TomCast

Alternaria

Alternaria solani occasionally causes leaf spot on pepper foliage. Alternaria alternata may cause fruit rot, particularly following sun scald or blossom end rot. Sun scald on pepper fruit usually occurs when the foliage is sparse and the peppers are exposed to sunlight. The injury becomes tan and shrunken and may appear water-soaked. When Alternaria colonizes these lesions they become chocolate brown to black and the fungus may be evident by a felty, dark brown to black growth. Alternaria fruit rot may also occur post-harvest.

The fungus infects stems, leaves and fruits. It may girdle seedlings causing damping-off in the seedbed. On the leaves, brown circular spots are often surrounded by a yellow area. Leaf spots have characteristic dark concentric rings. Leaf spots usually appear on the oldest leaves first and progress up the plant. As the disease progresses, the fungus may infect the stems and fruit. The spots on the fruit look similar to those on the leaves–brown with dark concentric rings. Dark, dusty spores are produced in concentric rings. The spores can be seen if the spot is touched to a light-colored object.

The fungus can survive in soil and in infested crop and weed residues. It may be seed-borne and carried by wind, water, insects, workers and farm equipment. The spores that land on plants will germinate and infect the leaves when they are wet. Spores can enter the leaf, steam or fruit. The fungus is most active during mild to warm temperatures and wet weather. The disease is worse during the rainy season. Early blight is most severe on plants stressed by a heavy fruit load, nematode attack, or low nitrogen fertility.

The susceptibility of the most pepper varieties to Alternaria is very low. Therefore using the TomCast model needs to adopt the action threshold to the specific susceptibility of the cultivar. Eggplant is susceptible to Alternaria. For this plant the thresholds, originally used in tomato are more propriate.

TomCast Model

The TomCast model is designed to evaluate the data of the first needed spray and the needed spray intersept. It is calculated on base of hours with leaf wetness (or relative humidity more than 90%) and the average temperature during this period. Every day is evaluated for this and every day gets a severity figure in between 0 and 4.

To assess the date of the first spary and the date when a spray has to be repeated the accumulated severity values are used. On the normally low susceptible pepper cultivars a high value (40 and more) can be accepted. After an injury like hail susceptibility is much higher and the acculated severity values have to be reduced to 18 to 25. This threshold has to be applied in Eggplants too.

Output in Fieldclimate: Severity values are accumulated regarding to favourable conditions of leaf wetness and temperature within this period (see tabel above). In Sum 24 Severity values have been accumulated within the observed time span of 10th of July to 27nd of August. In dependence of the cultivar (susceptibility) and of injuries the severity value a spray application is recommended at about 40 SV (not susceptible cultivar, no injuries) or at about 20 SV, when injuries are observed through hail or the cultivar is very susceptible to the pathogen.

Powdery mildew risk model

Powdery Mildew

Powdery mildew primarily affects leaves on pepper plants. Although the disease commonly occurs on older leaves just before or at fruit set, it can develop at any stage of crop development. Symptoms include patchy, white, powdery growth that enlarges and coalesces to cover the entire lower leaf surface. At times the powdery growth is present on the upper leaf surface as well. Leaves with mildew growing on the under surface may show a patchy yellowish or brownish discoloration on the upper surface. The edges of infected leaves may roll upwards exposing the white, powdery fungal growth. Diseased leaves drop from the plants and leave the fruit exposed to the sun, which may result in sunburning.

Powdery mildew can be severe during the warmest part of summer and can cause heavy yield losses. The pathogen has a very wide host range and inoculum from one host plant species can cross infect other host plants. In California, powdery mildew inoculum can come from crops such as onion, cotton, tomato, all varieties of peppers, and weeds such as annual sow thistle and ground cherry. This powdery mildew pathogen differs from powdery mildew pathogens in other genera in that it primarily occurs inside the leaf rather than on the leaf surface. Cleistothecia (sexual spores) of the Leveillula perfect stage rarely occur in California, but asexual spores (conidia) are produced and disseminated by wind. In general, high humidity favors germination of conidia. Infection of plants can occur over a wide temperature range (64° to 91°F or 18° to 33°C) under both high and low humidity. Under favorable conditions, secondary infections occur every 7 to 10 days, and disease can spread rapidly. Temperatures over 95°F that commonly occur in the interior valleys of the state can temporarily suppress development.

Risk Model

if temperature is in between 22°C and 32°C for 6 hours or more => Risk increases by 20 Points
if temperature is higher than 32°C for 6 hours or more => Risk decreases by 10 Points
if temperature is lower than 22°C for the whole day => Risk decreases by 10 Points
if there is more than 6 hours of leaf wetness => Risk decreases by 10 Points

If the risk is below 20 it can be assumed that powdery mildew can not propagate fast and the spry program can by very extensive. If the risk is higher than this the spray program should start. if the risk exceeds 60 points a strict and effective spray program has to be used. In organic growing this will include the reduction of the spray intercelt for sulfor.

In FieldClimate the risk is displayed in daily values. Because of the temperatures of 20 to 30°C at the beginning of August we have a risk of 100%.

Grey mould

Botrytis cinerea is a fakultative parasite, because it grows on dead plant material too. Because of this fact it is always present in vineyards, orchards and permanent land. Botrytis cinerea is related to moist climate. Infection takes place at very high relative humidity or the presence of free water. The fungus is unable to infect healthy adult plant material by spores. Infection takes place on young shoots during longer wet periods or an shoots damaged by hail storms.

Botrytis cinerea infection is favoured by leaf wetness. For the infection of detached berries the relation between leaf wetness duration and temperature leading to infection is shown in the graph above. This findings coming out a study performed at the UC-Davis. Pessl Instruments is using this relation as the basis for our B. cinerea risk model. Increase of risk is proportional to this graph and three completed infection periods would lead to a risk of 100%. Any leaf wetness period will lead to an increase of risk which is proportional to this. Days with less than 4 hours of leaf wetness will lead to a reduction of the risk by one fifth of the actual value.

Phytophthora blight

Phytophthora blight has become one of the most serious threats to production of cucurbits and peppers worldwide. Phytophthora capsici on cucurbits was first reported in 1937 in California and Colorado. Since then, Phytophthora blight has been observed in cucurbits and peppers in most of the vegetable producing areas in the world. Phytophthora capsici commonly occurs in temperate, subtropical, and tropical environments. Because P. capsici has a wide host range, it is difficult to control Phytophthora blight. More than 50 plant species, including several weed species, in more than 15 families are hosts for P. capsici. Among the major hosts of P. capsici are red and green peppers (Capsicum annuum), watermelon (Citrullum lanatus), cantaloupe (Cucumis melo), honeydew melon (C. melo), cucumber (Cucumis sativus), blue Hubbard squash (Cucurbita maxima), acorn squash (Cucurbita moschata), gourd (C. moschata), processing pumpkin (C. moschata), yellow squash (Cucurbita pepo), zucchini squash (C. pepo), tomato (Lycopersicon esculentum), black pepper (Piper nigrum), and eggplant (Solanum melongena). Currently, there is no single control method that is adequate to provide control of P. capsici on cucurbits. There are no cucurbit cultivars with measurable resistance to this disease. Crop rotations are virtually ineffective in controlling P. capsici because the pathogen can survive for several years in the soil, and it can infect more than 50 plant species. Outbreaks of the disease are seriously threatening the production of cucurbit crops (Figure 19). Additional research is needed to develop effective strategies for the management of Phytophthora blight on cucurbits and other vegetables.

Symptoms and Signs

Phytophthora blight is caused by the oomycete plant pathogen Phytophthora capsici. Infection of the plants are at any stage of development. The pathogen can infect seedlings, vines, leaves, and fruit. The infection usually appears first in low areas of the fields where soil remains wet longer.

Damping-off
Phytophthora capsici causes pre- and post-emergence damping-off in cucurbits under wet and warm 20-30°C (68-86°F) soil conditions. In seedlings, a watery rot develops on the hypocotyl at or near the soil line, resulting in plant death. Mature plants show symptoms of crown rot. Post-emergence plant death is preceded by plant wilting: a sudden, permanent wilt of the plant without a change in color of the foliage. Leaf wilting progresses from the base to the extremities of the vines. Plants often die within a few days of the first symptoms expression or after soil is saturated by excessive rain or irrigation. The stems of infected plants turn light to dark brown near the soil line and become soft and water-soaked. Infected stems collapse and die. The taproot and lateral roots of infected processing pumpkin plants usually do not exhibit symptoms. Following death of the foliage, roots may give rise to new vines if environmental conditions become less conducive for disease development. Phytophthora damping-off may result in partial to total loss of the crop.

Vine blight
Vines can be affected at any time during the growing season. Water-soaked lesions develop on vines. The lesions are dark olive and then become dark brown in a few days. Lesions girdle the stem, resulting in rapid collapse and death of foliage above the lesion.

Leaf symptoms
Phytophthora capsici can infect both the petioles and the leaf blades of plants. Dark brown, water-soaked lesions develop on petioles (similar to lesions on vines), resulting in rapid collapse of the petiole and leaf death. Infected leaf blades develop spots ranging from 5 mm (0.2 in.) to more than 5 cm (2 in.) in diameter. Infected areas are chlorotic at first, but within a few days they become necrotic with chlorotic to olive-green borders. Under wet and warm conditions, leaf spots expand rapidly, coalesce, and may cover the entire leaf. Under dry conditions, leaf spots cease to expand.

Fruit rot
Fruit rot can occur at any time from fruit set until harvest. Fruit rot generally starts on the site of the fruit that is in contact with the ground . However, occasionally infections will begin in other locations on the fruit where infected leaves or vines come into contact with a fruit. Also, symptoms on the upper surface of the fruit develop following rain or overhead irrigation, which can splash water containing the pathogen onto neighboring plants. Fruit rot also can develop after harvest, during transit or in storage. Fruit rot typically begins as a water-soaked lesion. Lesions expand, and become covered with white mold. The pathogen produces numerous sporangia on most infected fruit. Fruit infection progresses rapidly, resulting in complete collapse of the fruit. Phytophthora foliar blight and fruit rot may result in total loss of the crop.

Pathogen Biology

Phytophthora capsici is classified in the family Pythiaceae, order Peronosporales, and class Oomycetes. Oomycetes are not true fungi and have been placed in the kingdom Stramenopila. They are more closely related to brown algae than to true fungi. The pathogen produces asexual sporangia and biflagellate zoospores and sexual oospores. Mycelia are coenocytic (non-septate). Phytophthora capsici grows at 10 to 36°C (50 to 97°F), with optimal temperatures of 24 to 33°C (75-91°F). This pathogen grows rapidly on lima bean agar, and the colony diameter can reach up to 8 cm (3 in.) in 5 days. The growth patterns of colonies can vary from cottony, petaloid, rosaceous, to stellate (star-shaped. Sporangia (asexual fruiting bodies) of P. capsici are produced on sporangiophores (sporangia-producing hyphae) and are mostly papillate (having a small rounded protuberance). Sporangial shapes are influenced by light and other cultural conditions, and may appear as sub-spherical, ovoid, obovoid, ellipsoid, fusiform, or pyriform. The lengths and widths of sporangia can vary from 32.8 to 65.8 and 17.4 to 38.7 μm, respectively. Length/width ratios of sporangia range from 1.3:1 to 2.1:1. Sporangia have long pedicels (stalks), ranging from 35 to 138 μm. Pedicellate sporangia can be dispersed in wind driven rain. Under moist conditions, zoospores (asexual spores) are produced inside sporangia. Zoospores are single-celled and biflagellate. Phytophthora capsici also produces chlamydospores (thick-walled asexual spores), which may be terminal or intercalary (between cells) on the mycelium. Chlamydospores can range in diameter from 22 to 39 μm. Phytophthora capsici produces sexual structures called antheridia and oogonia, and sexual spores called oospores. Phytophthora capsici is predominantly heterothallic with two mating types known as A1 and A2. Antheridia are amphigynous (forming a collar at the base of the oogonium after the young oogonium grows through it), with diameters of 12–21 to 12–17 μm. Oogonia are spherical or sub-spherical, with diameters ranging from 23 to 50 μm. Oospores are predominantly plerotic (filling the oogonium) with wall thicknesses ranging from 2 to 6 μm, and diameters ranging from 22 to 35 μm. Phytophthora capsici is distinguished from other Phytophthora species by its sporangial morphology. Sporangia of P. capsici are caducous (easily separated from sporangiophores), have long pedicels, and are spherical to elongate with a tapering base. Significant differences in virulence (degree of pathogenicity) and genetics among isolates of P. capsici have been reported. Several methods can be used to study the genetic variation of P. capsici and other fungi. Sequencing and/or restriction digest of internal transcribed spacers (ITS) regions can be used for species identification. A specific PCR primer (Pcap) has been developed that can be used with iTS primers to specifically amplify P. capsici. Inter-simple sequence repeats (ISSR) amplification, amplified fragment-length polymorphism (AFLP), allozyme genotyping, and restriction fragment length polymorphisms with a probe can be used to study genetic variation among populations of P. capsici.

Disease Cycle and Epidemiology

Phytophthora capsici is a soilborne pathogen and survives between crops as oospores in soil or mycelium in plant debris. Oospores are resistant to desiccation, cold temperatures, and other extreme environmental conditions, and can survive in the soil, in the absence of a host plant, for several years. Oospores germinate and produce sporangia and zoospores. Zoospores are released in water and dispersed by irrigation or surface water. Zoospores are able to swim for several hours and infect plant tissues. Zoospores first lose their flagella and then encyst and form a cell wall, germinate and infect plant tissues. Abundant sporangia are produced on infected tissues, particularly on affected fruit. Sporangia are dispersed by water or in wind-driven rain in the air. Sporangia may either germinate directly and infect the host plant or germinate and give rise to zoospores that are released in water and infect the plant. The pathogen grows within the host and produces sporangia on the surface of the infected tissues. If the environmental conditions are conducive, the disease develops rapidly. Although the pathogen produces chlamydospores on culture media, their role in pathogen survival and diseases epidemiology is not known. Soil moisture conditions are important for disease development. Sporangia form when soil pores are drained, and they release zoospores when soil is saturated (soil pores are filled with water). The disease is usually associated with heavy rainfall, excessive-irrigation, or poorly drained soil. Frequent irrigation increases the incidence of the disease. Warm conditions are favorable for disease development.

Disease Management

No single method is available to provide adequate control of Phytophthora blight. Various disease control practices can be integrated to manage Phytophthora blight, including: exclusion, cultural practices, and chemical control. The most effective method of control for Phytophthora blight is to prevent P. capsici from moving into a non-infested field. Phytophthora capsici spreads by soil, water, and/or plant material. It is highly recommended to thoroughly clean all farm equipment that is used in an infested field before moving it to another field. Also, avoid using water sources (i.e. ponds or reservoirs) that receive run-off water from an infested field. Water sources can be tested for the presence of the pathogen by baiting techniques. Phytophthora capsici is not considered a seed-borne pathogen, however, saving seed from a field where Phytophthora blight occurred should be avoided.

Cultural Practices

The following cultural practices can help to manage Phytophthora blight in cucurbit fields. Because P. capsici can survive in soil for several years, fields without a history of Phytophthora blight should be selected for planting. Although no cropping rotation period has been established for effective management of Phytophthora blight of cucurbits, it is recommended to select only fields that have not had a history of cucurbits, eggplant, peppers, and/or tomatoes for at least 3 years. Fields should be selected that are well isolated from fields infested with P. capsici. High soil moisture favors the development of Phytophthora blight, thus well-drained fields should be selected and excessive irrigation should be avoided. Also avoid planting cucurbit crops in areas of the field that have poor drainage. Non-vining cucurbit crops (e.g. summer squash) should be planted on dome-shaped raised beds approximately 25 cm (10 in. high). The field should be scouted regularly for Phytophthora symptoms, especially after major rainfalls, and particularly in low areas of the field. When symptoms are localized in a small area of the field, the infected plants should be plowed into the soil. Plants should be sprayed with effective fungicides at the first sign of the disease. Healthy fruit should be removed from the infested area as soon as possible, and they should be checked for disease development routinely. Growing cover crops and/or mulching with plant materials including straw and rye vetch can also be used to manage the dispersal of the pathogen.

FieldClimate

In FieldClimate a model display the development of the fungal diseases: Model A: Model Phytophtora Capsici This model show infection rates on the base of precipitation, air temperature, leaf wetness duration and relative humidity. Two different forms of infections are shown:

Infection by Macrosporangia (Zoospores), which are dispersed by water (heavy rainfalls in soil)
Infection by Sporangia, which are dispersed by wind and water

Soil moisture conditions are important for disease development. Sporangia form when soil pores are drained, and they release zoospores when soil is saturated (soil pores are filled with water). The disease is usually associated with heavy rainfall, excessive-irrigation, or poorly drained soil. Frequent irrigation increases the incidence of the disease. Warm conditions are favorable for disease development.

Literature:

Babadoost, M. and S.Z Islam. 2003. Fungicide seed treatment effects on seedling damping-off of pumpkin caused by Phytophthora capsici. Plant Dis. 87:63-68.
Erwin, D.C. and O.K. Ribeiro. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society Press, St. Paul, MN.
Hausbeck, M. K., and K.H. Lamour. 2004. Phytophthora capsici on vegetable crops: Progress and challenges. Plant Dis. 88:1292-1303.
Islam, S.Z., M. Babadoost, K. Lambert, A. Ndeme, and H.M. Fouly. 2005. Characterization of Phytophthora capsici isolates from processing pumpkin in Illinois. Plant Dis. 89:191-197.
Lamour, K.H. and M.K. Hausbeck. 2000. Mefenoxam insensitivity and the sexual stage of Phytophthora capsici in Michigan cucurbit fields. Phytopathology 90:396-400.
Lee, B.K., B.S. Kim, S.W. Chang, and B.K. Hwany. 2001. Aggressiveness of isolates of Phytophthora capsici from pumpkin and pepper. Plant Dis. 85:797-800.
Leonian, L.H. 1922. Stem and fruit blight of pepper caused by Phytophthora capsici. Phytopathology 12:401-408.
Papavizas, G.S., J.H. Bowers, and S.A. Johnston. 1981. Selective isolation of Phytophthora capsici from soils. Phytopathology 71:129-133.
Ristaino, J.B. 1990. Interspecific variation among isolates of Phytophthora capsici from pepper and cucurbit fields in North Carolina. Phytopathology 80:1253-1259.
Ristaino, J. B. and S.A. Johnston. 1999. Ecologically-based approaches to management of Phytophthora blight on bell pepper. Plant Dis. 83:1080-1089.
Stamps, D.J. 1985. Phytophthora capsici. Commonw. Mycol. Inst. Descriptions of Pathogenic Fungi and Bacteria No. 836.
Stamps, D.J., G.M. Waterhouse, F.J. Newhook, and G.S. Hall. 1990. Revised tabular key to the species of Phytophthora. Commonw. Agric. Bur. Int. Mycol. Inst. Mycol. Pap. 162.
Tian, D. and M. Babadoost. 2003. Genetic variation among isolates of Phytophthora capsici from Illinois. Phytopathology 93:S84. Publication no. P-2003-0613-AMA.
Zitter, T.A., D.L. Hopkins, and C.E. Thomas. 1996. Compendium of Cucurbit Diseases. American Phytopathological Society Press, St. Paul, MN.

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